U.S. patent number 5,574,979 [Application Number 08/253,791] was granted by the patent office on 1996-11-12 for periodic interference avoidance in a wireless radio frequency communication system.
This patent grant is currently assigned to Norand Corporation. Invention is credited to Guy J. West.
United States Patent |
5,574,979 |
West |
November 12, 1996 |
Periodic interference avoidance in a wireless radio frequency
communication system
Abstract
A hierarchical communication system is described whereby
periodic interference can be detected and avoided utilizing
constituents' computer controllers and sync signals generated in
response to the interference. The use of a predictive sync signal
allows radio frequency communication to be optimally timed to
efficiently make use of the interference-free time within the
periodic interference signal.
Inventors: |
West; Guy J. (Cedar Rapids,
IA) |
Assignee: |
Norand Corporation (Cedar
Rapids, IA)
|
Family
ID: |
22961717 |
Appl.
No.: |
08/253,791 |
Filed: |
June 3, 1994 |
Current U.S.
Class: |
455/63.1;
375/356; 375/357 |
Current CPC
Class: |
H04B
1/1027 (20130101); H04W 52/46 (20130101); H04L
7/027 (20130101); H04L 7/046 (20130101); H04L
2007/045 (20130101) |
Current International
Class: |
H04B
1/10 (20060101); H04L 7/027 (20060101); H04L
7/04 (20060101); H04B 001/00 () |
Field of
Search: |
;455/63,67.5,50.1,51.1,9,54.1,67.3,67.1,67.6,295,296,298,299,226.1,218,222,223
;375/357,346,354,356 ;370/95.1 ;219/715,716 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Eisenzopf; Reinhard J.
Assistant Examiner: Nguyen; Lee
Attorney, Agent or Firm: McAndrews, Held & Malloy
Ltd.
Claims
What is claimed is:
1. A system for transmitting and receiving radio frequency
communication in an environment where there is periodic
interference, comprising:
(a) means for transmitting a communications message via wireless
radio frequency communication;
(b) sync means comprising:
(b1) sensing means for sensing periodic interference; and
(b2) means, coupled with said sensing means, for generating a sync
signal that tracks an alternating current associated with a power
main which is representative of the timing of said periodic
interference and controlling said transmitting means to effect
radio frequency transmission of a communication message according
to the sync signal, so as to tend to avoid errors in radio
frequency communication due to said periodic interference.
2. The system of claim 1, comprising a user supported terminal and
a radio base station, each having sync means and means for
transmitting and receiving communication messages via wireless
radio frequency communication.
3. The system of claim 2, wherein the user supported terminal and
the radio base station each include sync circuit means and
controlling means, comprising a computer controller, for
controlling transmission of communication messages to tend to avoid
errors in radio frequency communication due to said periodic
interference.
4. The system of claim 3, wherein the respective sync circuit means
of the user supported terminal and the radio base station is
coupled to the respective computer controller means of the user
supported terminal and the radio base station to supply the
respective sync signal thereto, so that two way transmission of
communications messages tends to avoid errors due to periodic
interference.
5. The system of claim 4, wherein the computer controller means of
the user supported terminal and the radio base station determine
whether to inhibit radio frequency communication based on
communication error rates.
6. The system of claim 1, wherein radio frequency communication
occurs in the 2.4 gigahertz band.
7. The system of claim 1, wherein the sync signal has first and
second logic levels corresponding to periodic interference, and
said transmitting means is controlled to effect radio frequency
communication a relatively short fixed time after the sync signal
shifts to the second logic level corresponding to an absence of
interference, and to inhibit radio frequency communication a
relatively short fixed time before the sync signal shifts to the
first logic level corresponding to the presence of
interference.
8. The system of claim 1, wherein the sync means includes a sync
waveform generator means for generating the sync signal, and the
sync waveform generator means is an analog integrated circuit.
9. The system of claim 1, wherein the sensing means comprises a
receiving loop means for sensing said periodic interference.
10. The system of claim 1, wherein the periodic interference is
generated by ac powered microwave oven magnetrons.
11. The system of claim 1, wherein the sensing means senses said
periodic interference by coupling directly to an ac power line.
12. A method of transmitting and receiving wireless communication
in an environment where there is periodic and non-periodic
interference, comprising the steps of:
sensing interference;
generating a sync waveform that tracks an alternating current
signal associated with a power main;
determining whether the sensed interference tracks the sync
waveform; and
selectively conducting wireless communication if the sensed
interference tracks the sync waveform, so as to tend to avoid
communication errors due to said periodic interference.
13. The method of claim 12, wherein wireless communication
comprises radio frequency transmission in the 2.4 gigahertz
band.
14. The method of claim 12, wherein the step of generating a sync
waveform is performed by an integrated circuit.
15. The method of claim 12, wherein the step of sensing
interference is performed utilizing a receiving loop.
16. The method of claim 12, wherein the periodic interference is
generated by a microwave oven.
17. The method of claim 12, wherein the step of sensing
interference is performed by coupling with the power main.
18. A wireless communication device comprising:
a receiver that wirelessly receives messages and detects
interference;
a transmitter that wirelessly transmits messages;
a synchronization circuit that tracks an alternating current cycle
associated with a power main;
a controller which interacts with said receiver and said
transmitter pursuant to a communication protocol for managing
exchanges of messages;
said controller coordinates with said synchronization circuit and
said receiver to identify periodic interference that is synchronous
with the alternating current cycle associated with the power main;
and
said controller adapting said protocol for operation when periodic
interference is identified.
19. The wireless communication device of claim 18 wherein the
period interference is generated by an external device that
connects to the power main.
Description
INCORPORATION BY REFERENCE
PCT Application Ser. No. PCT/US94/05037 (Attorney Docket No.
37998XAX) filed May 6, 1994, which is to be published in November,
1994, is incorporated herein by reference in its entirety,
including drawings and appendices, and hereby is made a part of
this application.
TECHNICAL FIELD
The present invention relates generally to local area networks used
for transmitting and receiving information and more particularly to
a singular radio using multiple communication protocols for
servicing corresponding multiple radio local area networks and to a
periodic noise detection and avoidance system operating within a
network environment.
BACKGROUND OF THE INVENTION
Multiple radio base station networks have been developed to
overcome a variety of problems with single radio base station
networks such as spanning physical radio wave penetration barriers,
wasted transmission power by portable computing devices, etc.
However, multiple radio base station networks have their own
inherent problems. For example, in a multiple base station network
employing a single shared channel, each base station transmission
is prone to collision with neighboring base station transmissions
in the overlapping coverage areas between the base stations.
Therefore, it often proves undesirable for each base station to use
a single or common communication channel.
In contradistinction, to facilitate the roaming of portable or
mobile devices from one coverage area to another, use of a common
communication channel for all of the base stations is convenient. A
roaming device may easily move between coverage areas without loss
of connectivity to the network.
Such exemplary competing commonality factors have resulted in
tradeoff decisions in network design. These factors become even
more significant when implementing a frequency hopping spread
spectrum network. Frequency hopping is a desirable transmission
technique because of its ability to combat frequency selective
fading, avoid narrowband interference, and provide multiple
communications channels.
Again, however, changing operating parameters between coverage
areas creates difficulties for the roaming devices which move
therebetween. In particular, when different communication
parameters are used, a portable or mobile device roaming into a new
base station coverage area is not able to communicate with the new
base station without obtaining and synchronizing to the new
parameters. This causes communication backlog in the network.
Moreover, even when a radio frequency network is established to
cover the premises of a building or group of buildings, certain
types of communication flow between certain types of devices make
for inefficient use of such a network. In fact, an ordinarily
efficient network configuration may be deemed intolerable in
certain communication scenarios.
Computer terminals and peripheral devices are widely used. Many
types of computer terminals exist which vary greatly in terms of
function, power and speed. Many different types of peripheral
devices also exist, such as printers, modems, graphics scanners,
text scanners, code readers, magnetic card readers, external
monitors, voice command interfaces, external storage devices, and
so on.
Computer terminals have become dramatically smaller and more
portable, as, for example, lap top computers and notebook
computers. Computer terminals exist which are small enough to be
mounted in a vehicle such as a delivery truck or on a fork lift.
Hand held computer terminals exist which a user can carry in one
hand and operate with the other.
Typical computer terminals must physically interface with
peripheral devices. Thus, there must either be a cable running from
the computer terminal to each peripheral device, or the computer
terminal must be docked with the device while information transfer
takes place.
In an office or work place setting, the physical connection is
typically done with cables. These cables pose several problems. For
example, many cables are required in order for a computer terminal
to accommodate many peripheral devices. In addition, placement of
peripheral devices is limited by cable lengths. While longer cables
may be used, they are costly. Additionally, there may be a limited
number of ports on a computer terminal, thus limiting the number of
peripherals that may be attached.
Another problem arises when several computer terminals must share
the same peripheral device, such as a printer. All of the computers
must be hardwired to the printer, which may create a protocol
problem if the computer terminals are of different types.
Peripheral cabling is an even greater problem in scenarios where
hand-held and portable computer terminals are used. The cabling
required for an operator to carry a hand-held computer terminal in
one hand, have a small portable printer attached to his belt, and
carry a code reader in the other hand is cumbersome and potentially
even dangerous. For example, such an operator loses a great deal of
mobility and flexibility while supporting a number of cabled
devices. In addition, as cables wear out and break, exposed
electric current could shock the operator, or create a spark and
potentially cause a fire or explosion in some work areas.
The requirement of physically connecting the computer terminals and
peripherals severely reduces the efficiency gained by making the
devices smaller. An operator must somehow account for all of the
devices in a system and keep them all connected. This can be very
inconvenient. For example, an operator having a notebook computer
and a modem in a briefcase may wish to have the freedom to move
around with the computer but without the modem. He may, for
example, wish to work at various locations on a job sight and at
various times transmit or receive information via his modem. If the
modem and the computer are hard wired together, he must either
carry the modem with him or keep connecting and disconnecting
it.
Furthermore, cabling can be expensive because cables frequently
prove to be unreliable and must be replaced frequently. In portable
environments, cables are subject to frequent handling, temperature
extremes, dropping and other physical trauma. It is not uncommon
for the cables or the connectors for the cables on the devices to
need replacing every three months or so.
Attempts to alleviate or eliminate these problems have been made
but have not been entirely successful. One solution is to
incorporate a computer terminal and all of the peripherals into one
unit. However, this solution proves unsatisfactory for several
reasons. For example, the incorporation of many devices into one
unit greatly increases the size and weight of the unit, thus
jeopardizing its portability. Furthermore, incorporating all of the
functions into one unit greatly reduces and, in most cases
eliminates, the flexibility of the overall system. A user may only
wish to use a hand-held computer terminal at times, but at other
times may also need to use a printer or occasionally a code reader.
An all-incorporated unit thus becomes either overly large because
it must include everything, or very limiting because it does not
include everything.
Another solution has been to set up Local Area Networks (LAN's)
utilizing various forms of RF (Radio Frequency) communication. The
LAN's to date, however, have been designed for large scale wireless
communications between several portable computer terminals and a
host computer. Therein, the host computer, itself generally a
stationary device, manages a series of stationary peripherals that,
upon requests to the host, may be utilized by the portable
terminals. Other large scale wireless communications have also been
developed which provide for RF communication between several
computer terminals and peripheral devices, but have proven to be
ineffective as an overall solution. For example, these systems
require the peripheral devices to remain active at all times to
listen for an occasional communication. Although this requirement
may be acceptable for stationary peripheral devices receiving
virtually unlimited power (i.e., when plugged into an AC outlet),
it proves detrimental to portable peripherals by unnecessarily
draining battery power. Similarly, in such systems, the computer
terminals are also required to remain active to receive an
occasional communication not only from the other terminals or the
host, but also from the peripherals. Again, often unnecessarily,
battery power is wasted.
In addition, such large scale systems are designed for long range
RF communication and often require either a licensed frequency or
must be operated using spread spectrum technology. Radios in such
systems are typically cost prohibitive, prove too large for
convenient use with personal computers and small peripheral
devices, and require a great deal of transmission energy
utilization.
Furthermore, these systems do not provide for efficient
communication between portable computer devices and peripherals.
For example, a portable computer device may be mounted in a
delivery truck and a driver may desire to transmit data to, or
receive data from, a host computer or peripheral device at a remote
warehouse location. While permitting such transmission, such wide
area networks (WANs) only provide point-to-point communications,
use a narrow bandwidth, and often have heavy communication traffic.
As a result, WANs are generally slow and expensive and simply do
not provide an effective overall solution.
Additionally, in order for a computer device to be effectively
portable in these systems, it must be capable of participating on
any number of LANs operating with different communication
parameters and protocols. Thus, each portable computer device
requires a plurality of built-in radio transceivers, one to
accommodate each of such LANs. As a result, portable computer
devices can become costly, excessively large, heavy, and power
hungry.
A further source of inefficiency in a LAN environment is periodic
interference, attributable to microwave ovens or other sources. A
standard interference avoidance protocol can detect periodic
interference as it occurs, through Received Signal Strength
Indicator (RSSI) and error rate monitoring, but a standard protocol
system cannot predict the future timing of a periodic interference
signal.
Thus, there is a need for a radio frequency communication system
and associated protocol that monitors RSSI and error rates to
detect the presence of interference, and which additionally can
detect when such interference is periodic in nature, so that the
system can predict the future timing of the periodic interference
and optimize communication procedures on the basis of such a
prediction.
An object of the invention is to provide a radio frequency
communication system that detects interference, and determines
whether such interference is periodic in nature.
Another object of the invention is to provide a radio frequency
communication system that detects interference and determines
whether it is periodic in nature by monitoring RSSI and error
rates.
A further object of the invention is to provide a radio frequency
communication system that responds to a periodic interference
signal by optimizing communication procedures based on a prediction
of the future timing of such a periodic interference signal.
SUMMARY OF THE INVENTION
The present invention solves many of the foregoing problems in a
variety of embodiments. The network and associated radio provide
wireless peripheralization of roaming computing devices and data
collection devices. The roaming computing devices communicate over
an extended area via a first local area network. The first local
area network is a high power radio communications system. Each
mobile computer device can communicate with peripheral devices via
a second, low-power local area network. Additionally, the roaming
computing device, also called the parent device, and peripherals
may communicate within a limited area while moving within an
independent wireless network that provides coverage over a much
broader service area. Thus, the communications system comprises at
least two independent wireless networks, with the parent device
participating in the multiple networks by selectively processing
and controlling the flow of information among devices connected to
the multiple networks.
In some embodiments, roaming computing or data collection devices
and their peripherals communicate within a building or group of
buildings serviced by a wireless LAN, hereinafter called a premises
LAN. Devices that are constituents of the premises LAN may also
contain facilities (transceivers and protocols) for communicating
with their peripherals via the separate low power, short range
radio LAN, hereinafter a peripheral LAN or MicroLAN. The parent
device contains significant processing power, such that information
received from various peripherals and other user input means, such
as a keyboard attached to the parent unit, is combined to form a
message that is communicated over the premises LAN. Information
received from peripherals is selectively communicated over the
premises LAN in accordance with an application program, emulation
mode, or operating system resident in the parent device.
Likewise, information received from the premises LAN may be
processed and selectively forwarded to a peripheral. For example, a
record received through the premises LAN may be combined with
information obtained locally through another peripheral or user
input means and processed by a local application to generate an
invoice or shipping document that is then sent wirelessly to a
portable printer peripheral.
The parent device may also enable communication among peripheral
devices within the peripheral LAN service area. Such communication
may be either forwarded from source to destination peripherals
through the parent device, or directly exchanged (peer to peer
communications). The former occurs if the peripherals are within
communication range of the parent but not each other, or if other
system design constraints, such as power management dictate a
centralized coordination function for power management or data
transfer efficiency.
The disclosed network and associated radio are also capable of
operation within radio Wide Area Networks (WANs). Vehicular based
data communication is currently serviced by a variety of public and
common carrier radio WANs that provide connectivity to computer
resources anywhere in the world, such as RAM Mobile Data, ARDIS,
MTEL, data PCS, CDPD, SMR, etc. The radio WANs are generally
bandwidth limited, and users are charged for service on the basis
of the amount of data transferred. The hierarchical network
provides means of connecting a premises LAN with a group of remote
devices via a radio WAN so as to minimize the expense and delays of
such radio WANs.
For example, communication between a parent device or radio
terminal mounted in a vehicle, hereinafter a "vehicle ternfinal",
and hand-held mobile terminal(s) that roam in an area local to the
vehicle can be carried out using radio communication not subject to
air time fees. Hereinafter, the network formed by the vehicle
terminal and associated hand-held terminal(s) is referred to as a
"vehicular LAN".
The intelligence of the parent device, in the previous example a
vehicle terminal, is key in processing information generated
locally in the vehicular LAN and passing only essential information
through the radio WAN to minimize cost of use. Information that is
not time critical may be selectively processed and stored for later
batch downloading via the premises LAN using hard-wired modems or
wireless communication at a docking station located at a depot or
central office.
The autonomous operation of the vehicular LAN allows it to continue
to function when it is out of range of the WAN, when the WAN is
inaccessible during peak usage periods, or when economics dictate
that WAN communication is unjustified.
In addition, a roaming computing device may have a single radio
unit which has a control processor, memory, and a transceiver. The
transceiver is capable of participating in at least a first and
second local area network which operate using a first and second
communication protocol, respectively. The radio unit may
participate as a slave to the first network pursuant to the first
protocol and as a master to the second network pursuant to the
second protocol, and the control processor resolves conflicts
between the first and second protocols.
In a further embodiment, the control processor causes the radio
unit to enter a state of low power consumption when the radio unit
is not communicating on either the first or second network.
User supported terminals and radio base stations of the present
invention are further operable to detect the presence of
interference, for example by monitoring RSSI and error rates
associated with radio frequency communication. The control
processor of each device can determine whether the interference
detected is periodic in nature based on the timing of the RSSI and
error rates. Upon determining that an interference signal is
periodic, the user supported terminal and radio base station adjust
their communication procedures to effect radio frequency
communication when interference is absent, and to inhibit radio
frequency communication when interference is present.
One embodiment of the present invention provides a method for
dynamically adjusting the communication protocol such that radio
frequency communication time interval structures can "mold"
themselves to fit within the time space where interference is
absent. When a communication time interval is forced to cut off due
to an impending presence of interference, the adjusting protocol
can determine when it is most efficient to stop the communication,
and can continue the rest of the communication time interval
structure after the interference abates.
Other objects, advantages, and novel features of the present
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a diagrammatic illustration of a hierarchical
communication system built in accordance with the present
invention.
FIG. 1B is a diagrammatic illustration of another hierarchical
communication system built in accordance with the present
invention.
FIG. 1C is a diagrammatic illustration of still another
hierarchical communication system built in accordance with the
present invention.
FIG. 2 illustrates an embodiment of a basic access interval
structure used by a hierarchical network of the present
invention.
FIGS. 3A and 3B illustrate the frequency of operation periodically
changing corresponding to access interval boundaries in a frequency
hopping communication protocol of the present invention.
FIGS. 4A and 4B illustrate more than one access interval being used
per hop in a frequency hopping communication protocol of the
present invention.
FIG. 5A illustrates an embodiment of an access interval used by the
hierarchical network of the present invention wherein a reservation
phase is Idle Sense Multiple Access.
FIG. 5B illustrates an embodiment of an access interval used by the
hierarchical network of the present invention wherein a device
response follows a reservation poll.
FIG. 6A illustrates an embodiment of an access interval used by the
hierarchical network of the present invention having multiple
reservation slots for transmission of a Request For Poll
signal.
FIG. 6B illustrates an embodiment of an access interval used by the
hierarchical network of the present invention wherein general
devices contend for channel access.
FIG. 7A illustrates a sequence in an access interval used by the
hierarchical network of the present invention for transferring data
from a remote device to a control point device.
FIG. 7B illustrates a sequence in an access interval used by the
hierarchical network of the present invention for transferring data
from a control point device to a remote device.
FIG. 8 illustrates a preferred embodiment of an access interval
used by the hierarchical network of the present invention.
FIGS. 9A and 9B conceptually illustrate how multiple NETs may be
employed in an idealized cellular-type installation according to
the present invention.
FIG. 10 illustrates a base station coverage contour overlap for the
multiple NETs Infrastructured Network of FIG. 1.
FIG. 11 illustrates hopping sequence reuse in a multiple NET
configuration of the present invention.
FIG. 12 illustrates a hierarchical infrastructured network of the
present invention wherein a wireless link connects base stations on
separate hard wired LANs.
FIG. 13 illustrates a hierarchical infrastructured network of the
present invention including a wireless base station.
FIG. 14 illustrates conceptually base stations communicating
neighboring base station information to facilitate roaming of
portable/mobile devices.
FIG. 15 illustrates a secondary access interval used in the
MicroLAN or peripheral LAN in the hierarchical communication
network according to the present invention.
FIG. 16 is a flow chart illustrating the selection of a base
station by a mobile computing device for communication
exchange.
FIG. 17 is a flow chart illustrating a termnninal maintaining
synchronization with the network after it has gone to sleep for
several access intervals.
FIG. 18 is a flow chart illustrating a terminal maintaining or
achieving synchronization with the network after it has gone to
sleep for several seconds.
FIGS. 19A and 19B are flow charts illustrating an access interval
during inbound communication.
FIGS. 20A and 20B are flow charts illustrating an access interval
during outbound communication.
FIG. 21 illustrates a sequence in an access interval used in the
hierarchical communication network of the present invention with
Time Division Multiple Access slots positioned at the end of the
access interval.
FIG. 22 illustrates a sequence in an access interval used by the
hierarchical network of the present invention with the Time
Division Multiple Access slots positioned immediately following the
SYNC.
FIG. 23 illustrates a sequence in an access interval used by the
hierarchical network of the present invention with the Time
Division Multiple Access slots positioned immediately following the
SYNC and Reservation Poll.
FIG. 24 illustrates another sequence in an access interval used by
the hierarchical network of the present invention with the Time
Division Multiple Access slots positioned immediately following the
SYNC.
FIG. 25 illustrates a portion of an access interval including the
preamble, SYNC and Reservation Poll.
FIG. 26 illustrates the information contained in a sample SYNC
message.
FIG. 27 illustrates the information contained in a sample
Reservation Poll.
FIG. 28a illustrates a warehouse environment incorporating a
communication network which maintains communication connectivity
between the various network devices according to the present
invention.
FIG. 28b illustrates other features of the present invention in the
use of a vehicular LAN which is capable of detaching from the
premises LAN when moving out of radio range of the premises LAN to
perform a service, and reattaching to the premises LAN when moving
within range to automatically report on the services rendered.
FIG. 28c illustrate other features of the present invention in the
use of a vehicular LAN which, when out of range of the premises
LAN, is still capable gaining access to the premises LAN via radio
WAN communication.
FIG. 29 is a diagrammatic illustration of the use of a peripheral
LAN supporting roaming data collection by an operator according to
the present invention.
FIG. 30 is a block diagram illustrating the functionality of RF
transceivers built in accordance with the present invention.
FIG. 31 is a diagrammatic illustration of an alternate embodiment
of the peripheral LAN shown in FIG. 2.
FIG. 32 is a block diagram illustrating a channel access algorithm
used by peripheral LAN slave devices in accordance with the present
invention.
FIG. 33a is a timing diagram of the protocol used according to the
present invention illustrating a typical communication exchange
between a peripheral LAN master device having virtually unlimited
power resources and a peripheral LAN slave device.
FIG. 33b is a timing diagram of the protocol used according to the
present invention illustrating a typical communication exchange
between a peripheral LAN master device having limited power
resources and a peripheral LAN slave device.
FIG. 33c is also a timing diagram of the protocol used which
illustrates a scenario wherein the peripheral LAN master device
falls to service the peripheral LAN slave devices.
FIG. 34 is a timing diagram illustrating the peripheral LAN master
device's servicing of both the higher power portion of the premises
LAN as well as the lower power peripheral LAN subnetwork with a
single or plural radio transceivers.
FIGS. 35 and 36 are block diagrams illustrating additional power
saving features according to the present invention wherein ranging
and battery parameters are used to optimally select the appropriate
data rate and power level of subsequent transmissions.
FIG. 37 illustrates an exemplary block diagram of a radio unit
capable of current participation on multiple LANs according to the
present invention.
FIG. 38 illustrates an exemplary functional layout of the frequency
generator of FIG. 37 according to one embodiment of the present
invention.
FIG. 39 illustrates further detail of the receiver RF processing
circuit of FIG. 37 according to one embodiment of the present
invention.
FIG. 40 illustrates further detail of the receiver signal
processing circuit of FIG. 37 according to one embodiment of the
present invention.
FIG. 41 illustrates further detail of the receiver signal
processing circuit of FIG. 37 according to another embodiment of
the present invention.
FIG. 42 illustrates further detail of the memory unit of FIG. 37
according to one embodiment of the present invention.
FIG. 43 illustrates a software flow chart describing the operation
of the control processor in controlling the battery powered radio
unit to participate on multiple LANs.
FIG. 44 is an alternate embodiment of the software flow chart
wherein the control processor participates on a master LAN and,
when needed, on a slave LAN.
FIG. 45 is a diagrammatic illustration of exemplary basic structure
and components which may be utilized in a user supported terminal
and in a radio base station according to the present invention.
FIG. 46 illustrates in more detail the components of a user
supported terminal according to the present invention.
FIG. 47 illustrates in more detail the components of a radio base
station according to the present invention.
FIG. 48 is a schematic diagram of the sync generating circuit block
of FIGS. 47 and 48.
FIG. 49 is a diagram showing the timing relationship between a
power frequency ac coupled input waveform and a sync output
waveform generated therefrom, according to the present
invention.
FIG. 50 is a diagram illustrating an exemplary timing relationship
between an interference wave, a sync signal, and enabled radio
frequency communication according to the present invention.
FIG. 51 is a flow diagram illustrating the process for determining
whether to inhibit radio frequency communication in response to a
generated sync signal, according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1A illustrates a hierarchical communication system 10 within a
building. The illustrated hierarchical communication system 10
includes a local area network (LAN) for maintaining typical
communication flow within the building premises, herein referred to
as a premises LAN. The premises LAN is designed to provide
efficient end-to-end routing of information among hardwired and
wireless, stationary and roaming devices located within the
hierarchical communication system 10.
The premises LAN consists of an infrastructure network comprising
radio base stations 15 and a data base server 16 which may be part
of a more extensive, wired LAN (not shown). The radio base stations
15 may communicate with each other via hard-wired links, such as
Ethernet, RS232, etc., or via wireless (radio frequency) links. A
plurality of roaming terminal devices, such as a roaming computing
device 20, participate in the premises LAN of the hierarchical
communication network 10 to exchange information with: 1) other
roaming computing devices; 2) the data base server 16; 3) other
devices which might be associated with data base server 16 (not
shown); and 4) any other devices accessible via the premises LAN
(not shown). A roaming computing device can be, for example, a
hand-held computer terminal or vehicle mounted computer terminal
(vehicle terminal).
In most circumstances, the premises LAN provides a rather optimal
solution to the communication needs of a given network. However, in
some circumstances, to serve a variety of particular communication
needs, the premises LAN does not offer the optimal solution.
Instead of relying on the premises LAN for such communications,
when and where beneficial, alternate LANs are spontaneously created
by (or with) network devices, such as the roaming computing device
20, within the hierarchical communication system 10. Such
spontaneously created LANs are referred to herein as spontaneous
LANs. After the immediate benefits end, i.e., a task has been
completed, or if the participants of the spontaneous LAN move out
of range of each other, the spontaneous LAN terminates
operation.
An exemplary spontaneous LAN involves the use of peripheral devices
as illustrated in FIG. 1A. Although bulk data transfer destined for
a peripheral device 23, such as a printer, from the roaming
computing device 20 might be communicated through the premises LAN,
a more direct interconnection proves less intrusive, saves power,
and offers a lower cost solution. Specifically, instead of
communicating through the premise LAN, the roaming computing device
20 needing to prim: 1) identifies the presence of an available
printer, the peripheral device 23; 2) establishes an RF link
(binds) with the peripheral device 23; 3) directly begins
transferring the bulk data for printing; and 4) lastly, when the
roaming terminal finishes the transfer, the spontaneous LAN with
the peripheral device 23 terminates. A spontaneous LAN created
between the computing devices and peripheral devices is herein
referred to as a peripheral LAN. Other types of spontaneous LANs,
such as vehicular LANs, are also possible. Embodiments described
below identify vehicular LANs and wide area radio networks (WANs)
which are part of the hierarchical communication system according
to the present invention.
Although a spontaneous LAN may operate completely independent of
the premises LAN, it is more likely that there will be some degree
of coordination between the two. For example, while participating
in the peripheral LAN, the roaming computing device 20 may
terminate participation in the premises LAN, and vice versa.
Alternately, the roaming computing device 20 may only service the
peripheral LAN when specific participation on the premises LAN is
not required, or vice versa. Moreover, the roaming computing device
20 may attempt to service each peripheral LAN as necessary in a
balanced time-sharing fashion, placing little priority upon either
LAN. Thus, based on the protocols and hardware selected, a
spontaneous LAN can be configured so as to exist hierarchically
above, below, at the same level, or independent of the premises
LAN.
Generally, to design a given LAN configuration, only the
characteristics of that LAN are considered for optimization
purposes. However, in the hierarchical communication system of the
present invention, the operation of other LANs must also be taken
into account. For example, because of the roaming computing devices
participation in both the premises and peripheral LANs, the
requirements and operation of the premises LAN must be taken into
consideration when defining the peripheral LAN, and vice versa.
Thus, the hierarchical communication system of the present
invention provides a series of tightly coupled radio LANs and WANs
with radio transceiver and communication protocol designs which
take into consideration such factors as cost, weight, power
conservation, channel loading, response times, interference,
communication flow, etc., as modified by a primary factor of
multiple participation.
LAN replaces hard-wired connection between a roaming computing
device and associated peripherals. In a typical configuration, a
peripheral LAN will consist of one or more peripherals slaved to a
single master roaming computing device, although multiple master
roaming computing devices are possible. Peripheral devices may be
printers, code scanners, magnetic card readers, input stylus,
etc.
Each of the peripheral devices 22 has a built-in radio transceiver
to communicate with the roaming computing devices 20. The roaming
computing devices 20 are configured with built-in radio
transceivers capable of communicating on both the peripheral and
premises LAN. The radio base stations 15 may be configured with
radio transceivers only capable of communicating in the premises
LAN. In alternate embodiments, as described below, the radio base
stations 15 might instead be configured to participate on both the
premises and peripheral LANs.
In particular, the peripheral LAN is intended to provide
communications between two or more devices operating within near
proximity, e.g., distances of a few tens of feet. The majority of
constituents of the peripheral LAN are generally devices that do
not require access to resources outside their immediate group, or
which can suffice with indirect access through devices which
participate outside their immediate peripheral LAN group. In
contradistinction, the premises LAN is intended to provide
communications between relatively many devices operating across
great distances throughout a building.
The characteristics of the peripheral LAN permit the use of radio
transceivers of lower cost, lower power consumption, and generally
more simplistic operation than permitted by the premises LAN.
However, the operation of the peripheral LAN is adapted for
integration with the premises LAN so that a radio transceiver and
protocol designed for operation on the premises LAN includes
features which allow concurrent or sequentially concurrent
operation on the peripheral LAN. For example, by selecting similar
communication hardware characteristics and integrating protocols,
communication within the premises and peripheral LANs may be
achieved with a single radio transceiver.
In one embodiment, radio communication through the premises LAN,
i.e., among the radio base stations 15 and the roaming computing
device 20, utilizes relatively higher-power spread-spectrum
frequency-hopping communication with a reservation access protocol.
The reservation access protocol facilitates frequency-hopping and
supports adaptive data rate selection. Adaptive data rate selection
is based upon the quality of communication on the premises LAN
radio channel. Radio communication through the peripheral LAN
utilizes a relatively lower-power single frequency communication
also with a reservation access protocol. As more fully described
below, the coordinated use of reservation access protocols in the
peripheral and premises LANs maximize information flow while
minimizing conflicts between devices participating in the two
LANs.
Referring to FIG. 1B, a small hierarchical communication system 30
is shown. A base station 33 and two roaming or mobile computing
devices 35 and 36 form a premises LAN 37. The premises LAN 37
provides for communication among the mobile computing devices 35
and 36 and a host computer 34. The mobile computing devices 35 and
36 can roam anywhere within the range of the base station 33 and
still communicate with the host computer 34 via the base station
33.
Two peripheral LANs 40 and 41 allow for wireless communication
between each mobile computing device 35 and 36 and their respective
peripheral devices 43, 44 and 45 when the mobile computing device
is not communicating on the premises LAN 37. Specifically, the
peripheral LAN 40 consists of the mobile computing device 35 and
the peripheral device 43, while the peripheral LAN 41 consists of
the mobile computing device 36 and the two peripheral devices 44
and 45.
FIG. 1C illustrates a larger hierarchical communication system 50.
The host computer 55 is connected to base stations 56, 57, 58 and
59. The host computer 55 and the base stations 56, 57, 58 and 59
provide the infrastructure for the premises LAN. The base stations
need not be hard-wired together. For example, as illustrated in
FIG. 1C, the base stations 56, 57 and 58 access each other and the
host computer 55 via a hard-wired link, while the base station 59
accomplishes such access via a wireless link with the base station
58.
The base stations 56, 58 and 59 can support multiple mobile
computing devices. For example, the base station 56 uses a
frequency-hopping communication protocol for maintaining
communication with mobile computing devices 61 and 62. Moreover,
each of the mobile computing devices may roam out of range of the
base station with which they have been communicating and into the
range of a base station with which they will at least temporarily
communicate. Together, the host computer 55 and the base stations
56, 57, 58 and 59 and mobile computing devices 61, 62, 64, 65 and
66 constitute a premises LAN.
More particularly, each base station operates with a different set
of communication parameters. For example, each base station may use
a different frequency hopping sequence. Additionally, different
base stations may not employ a common master clock and will not be
synchronized so as to have the frequency hopping sequences start at
the same time.
Mobile computing devices 61, 62, 64, 65 and 66 are capable of
roaming into the vicinity of any of the base stations 56, 58 and 59
and connecting thereto. For example, mobile computing device 62 may
roam into the coverage area of base station 58, disconnecting from
base station 56 and connecting to base station 58, without losing
connectivity with the premises LAN.
Each mobile computing device 61, 62, 64, 65 and 66 also
participates with associated peripherals in a peripheral LAN. Each
peripheral LAN is made up of the master device and its slave
device. Similarly, as illustrated, the base station 57 is shown as
a direct participant in not only the premises LAN but also in the
peripheral LAN. The base station 57 may either have limited or full
participation in the premises LAN. For example, the base station 57
may be configured as a mobile computing device with the full RF
capability of transmission in both the premises and peripheral
LANs. Instead, however, participation in the premises LAN may be
limited to communicating through the hard-wired link, effectively
dedicating the base station 57 to the task of servicing
peripherals.
Although the use of a plurality of built-in radio transceivers
could be used so as to permit simultaneous participation by a
single device, factors of cost, size, power and weight make it
desirable to only build-in a single radio transceiver capable of
multiple participation. Furthermore, even where a plurality of
radio transceivers are built-in, simultaneous participation may not
be possible depending upon the potential transmission interference
between transceivers. In fact, full simultaneous participation may
not be desirable at least from a processing standpoint when one
transceiver, servicing one LAN, always or usually takes precedence
over the other. Justification for such precedence generally exists
in a premises LAN over a peripheral LAN.
For example, communication flow in most premises LANs must be fast,
efficient and rather robust when considering the multitude of
participants that operate thereon. In the peripheral LAN, however,
response times and other transmission related delays are generally
more acceptable--even adding extra seconds to a peripheral
printer's print time will usually not bother the user. Thus, in
such communication environments, it may be desirable to design the
transmitters and associated protocols so that the premises LAN
takes precedence over the peripheral LAN. This may yield a
communication system where fully simultaneous participation in both
the premises and peripheral LANs does not exist.
In communication environments wherein fully simultaneous
participation does not exist or is not desired, transmitter
circuitry might be shared for participation in both the premises
and peripheral LANs. Similarly, in such environments, the
communication protocol for the peripheral LAN can be tightly
coupled with the protocol for the premises LAN, i.e., integrated
protocols, so as to accommodate multiple participation. Moreover,
one protocol might be designed to take precedence over the other.
For example, the premises LAN protocol might be designed so as to
minimize participation or response time in the peripheral LAN. As
described in more detail below, such transceiver and protocol
analysis also takes place when considering additional multiple
participation in the vehicular LAN and WAN environments.
FIG. 2 illustrates an embodiment of a communication protocol for
the premises LAN which uses a basic Access Interval 200 ("AI")
structure according to the present invention. Generally, an Access
Interval is the basic communication unit, a fixed block of time,
that allocates bandwidth to synchronization, media access, polled
communications, contention based communications, and scheduled
services. The Access Interval in FIG. 2 includes a SYNC header 201
generated by a Control Point ("CP") device of a NET. The term NET
describes a group of users of a given hopping sequence or a hopping
sequence itself. The Control Point device is generally the base
station 15 referenced above with regard to FIG. 1. The SYNC header
201 is used by constituents of the NET to attain and maintain
hopping synchronization. A reservation phase 203 follows permitting
a reservation poll, which provides the NET constituents an
opportunity to gain access to media. A sessions frame 205 is next
allocated for communication protocol. A frame 207 follows for
optional time division multiple access ("TDMA") slots in order to
accommodate scheduled services. Scheduled services, for example,
real time voice or slow scan video, are such that they require a
dedicated time slot to provide acceptable quality of service. The
function of frames 201,203,205 and 207 will be discussed in greater
detail below.
As was shown in FIG. 2, FIG. 21 illustrates a sequence in an access
interval 2100 with the Time Division Multiple Access slots 2113
positioned at the end of the access interval 2100. In present
example, if this were also a HELLO interval, the HELLO would
immediately follow the SYNC 1201. Location of the Time Division
Multiple Access slots at such a position provides certain
advantages including, for example, 1) the SYNC 2101, HELLO (not
shown), Reservation Poll 2103, may all be combined into a single
transmission (concatenated frames); 2) hopping information may be
moved to or included in the Reservation Poll 2103 allowing for a
shorter preamble in the SYNC 2101; and 3) the HELLO messages will
occur early in the Access Interval 2100 providing for shorter
receiver on times for sleeping terminals.
The Time Division Multiple Access slots may also be located at
different points within the access interval. Positioning the Time
Division Multiple Access slots allow for various systemic
advantages. Referring now to FIG. 22, an access interval 2200 is
illustrated showing the Time Division Multiple Access slots 2203
immediately following the SYNC 2201. Location of the Time Division
Multiple Access slots 2203 at this position provides certain
advantages including, for example, 1) better timing accuracy is
achieved when the Time Division Multiple Access slots 2203
immediately follow the SYNC 2201; 2) Session Overruns do not
interfere with the Time Division Multiple Access slots 2203; 3)
devices which do not use the Time Division Multiple Access slots
2203 do not necessarily need to be informed of the Time Division
Multiple Access slot allocation; and 4) HELLO message may follow
Time Division Multiple Access slots 2203, Reservation Slots 2207 or
Reservation Resolution Poll 2209.
Referring now to FIG. 23, an access interval 2300 is illustrated
showing the Time Division Multiple Access slots 2305 immediately
following the SYNC 2301 and the Reservation Poll 2303. In the
present example, if this were a HELLO interval, a HELLO message
would immediately follow the Reservation Resolution Poll 2309.
Location of the Time Division Multiple Access slots 2305 at the
position shown in FIG. 23 provides certain advantages including,
for example, 1) the Time Division Multiple Access slot timing is
keyed to SYNC 2301 for better accuracy; 2) the number of Time
Division Multiple Access slots 2305 may be indicated in SYNC 2301
or the Reservation Poll 2303, providing greater flexibility; 3)
Session frame overruns do not interfere with Time Division Multiple
Access slots 2305; 4) only one maintenance transmission is required
per Access Interval 2300; and 5) hopping information may be moved
to or included in the Reservation Poll 2303, permitting a shorter
preamble in SYNC 2301.
In the access interval 2300 configuration shown in FIG. 23, it is
possible that the Time Division Multiple Access slots 2305 and the
response slots 2307 could be the same. The Reservation Poll 2303
would allocate the correct number of slots and indicate which are
reserved for Time Division Multiple Access. For example, to use
Idle Sense Multiple Access 1 slot) with 1 inbound and 1 outbound
Time Division Multiple Access slots, three slots would be allocated
with the first two slots reserved. The appropriate Time Division
Multiple Access slot duration is 80 bits at a hop rate of 200 hops
per second which is just about the expected duration of a Request
for Poll. At slower hop rates, multiple slots could be allocated to
Time Division Multiple Access allowing the Time Division Multiple
Access slot duration to be constant regardless of hop rate.
Referring now to FIG. 24, another access interval 2400 is
illustrated showing the Time Division Multiple Access slots 2403
immediately following the SYNC 2401. In this example the Poll
Message Queue 2405 immediately follows the Time Division Multiple
Access slots 2403. The configuration shown in FIG. 24 provides for
certain advantages including, for example, 1) the Time Division
Multiple Access slot timing is keyed to SYNC 2401 for better
accuracy; and 2) Session frame overruns do not interfere with Time
Division Multiple Access slots 2403.
The configurations shown in FIG. 21 and in FIG. 23 are preferred
because they allow the Reservation Poll messages to be transmitted
immediately following the SYNC and because of the power management
and interference reduction advantages.
In one embodiment of the Access Interval structure, all message
transmissions use standard high-level data link control ("HDLC")
data framing. Each message is delimited by High-Level Data Link
Control Flags, consisting of the binary string 01111110, at the
beginning of the message. A preamble, consisting of a known data
pattern, precedes the initial FLAG. This preamble is used to attain
clock and bit synchronization prior to start of data. Receiver
antenna selection is also made during the preamble for antenna
diversity. A CRC for error detection immediately precedes the
ending FLAG. Data is NRZ-I (differentially) encoded to improve data
clock recovery. High-Level Data Link Control NRZ-I data is
run-length-limited to six consecutive bits of the same state.
Alternatively, a shift register scrambler could be applied instead
of differential encoding to obtain sufficient transitions for clock
recovery. Data frames may be concatenated, with two or more frames
sent during the same transmission, with a single FLAG separating
them. An example of this is SYNC, followed by a HELLO or
Reservation Poll (SYNC, HELLO and Reservation Poll are discussed
more fully below).
While much of the following discussion centers on the use of
frequency hopping in the premises LAN, the Access Interval
structure of the present invention is also suitable for single
channel and direct sequence spread spectrum systems. The consistent
timing of channel access, and the relative freedom from collisions
due to channel contention, provide desirable benefits in systems
that support portable, battery powered devices regardless of
modulation type or channelization. Functions that are unique to
frequency hopping may be omitted if other channelization approaches
are used.
FIGS. 3a and 3b illustrate the frequency of operation periodically
changing corresponding to Access Interval boundaries in a frequency
hopping system. Frequency hopping systems use a hopping sequence,
which is a repeating list of frequencies of length (n) selected in
a pseudo random order and is known to all devices within a coverage
area. FIG. 3a illustrates a frequency hopping system having one
Access Interval 301 per frequency hop (the hop occurring every 10
milliseconds) and a length of 79. FIG. 3b illustrates a frequency
hopping system having one Access Interval 303 per frequency hop
(the hop occurring every 20 milliseconds) and a length of 79
frequencies. The 20 ms time frame is preferred for a protocol stack
that uses a maximum network layer frame of up to 1536 bytes payload
while maintaining two real time voice communications channels.
Access interval duration may be optimized for other conditions.
Access Interval length is communicated to the NET during the SYNC
portion of the Access Interval. This allows Access Interval
duration, and other NET parameters to be adjusted without
reprogramming every device within the NET.
The Access Interval is a building block. The length of the Access
Interval can be optimized based on network layer packet size,
expected mix of Bandwidth on Demand ("BWOD") and Scheduled Access
traffic, expected velocities of devices within the NET, acceptable
duration of channel outages, latency or delay for scheduled
services, periodic interference, etc. The preferred Access Interval
duration of 20 ms (and maximum packet length of 256 Bytes at 1
MBIT/sec) represents a value chosen for systems with device
velocities up to 15 MPH, and a mix between Bandwidth On Demand and
scheduled service traffic.
Within a frequency hopping network, one or more Access Intervals
may be used during each dwell in a frequency hopping system. A
dwell is the length of time (d) each frequency in the hopping
sequence is occupied by the system. For example, FIGS. 4a and 4b
show illustrations of cases where more than one 20 ms Access
Interval 401 is used per hop. This may be appropriate for some
instances where it is undesirable to hop at higher rates because of
relatively long frequency switching times of the radio hardware,
where import, export, or regulatory restrictions disallow hopping
at a faster rate, or in some applications where it is desirable to
maintain operation on each channel for a longer period. An example
of the latter is the case where larger files or data records are
transferred routinely.
In a frequency hopping operation, the Access Interval 200 of FIG. 2
begins with a SYNC header 201. As mentioned above, the SYNC is
generated by the Control Point (CP) device of the NET. The SYNC is
used by constituents of the NET to attain and maintain hopping
synchronization. Included in the SYNC are:
1. Address of the Control Point device.
2. Identification of the Hopping Sequence, and index of the current
frequency within the hop table.
3. Identification of the hop rate, number of Access Intervals per
hop, and Access Intervals before next hop.
4. A timing character for synchronization of device local clocks to
the NET clock contained within the Control Point device.
5. Status field indicating reduced SYNC transmissions due to low
NET activity (Priority SYNC Indicator).
6. Status field indicating if the Access Interval will contain a
broadcast message to all devices within the NET.
7. Status field indicating premises or spontaneous LAN
operation.
8. The SYNC field information is optionally encrypted using a block
encryption algorithm, with a key provided by the network user. A
random character is added to each SYNC message to provide
scrambling.
However, there are two circumstances during which a SYNC message is
not transmitted: 1) co-channel interference; and 2) low NET
utilization. With regard to co-channel interference, before issuing
a SYNC message, the Control Point device performs channel
monitoring for a brief interval. If the Received Signal Strength
Indicator (RSSI) level indicates an ON channel signal greater than
the system defer threshold, then the Access Interval is skipped.
Alternatively, a strong ON channel signal may dictate a reduction
in Control Point device power to limit the interference distance of
the net for the duration of the Access Interval. A system defer
threshold 30 dB above the receiver sensitivity is a preferred
choice. Communication within the NET is deferred for the duration
of the Access Interval if SYNC is not transmitted due to co-channel
interference.
In times of low system utilization, SYNC and Reservation Poll
messages are reduced to every third Access Interval. The SYNC
message includes a status field indicating this mode of operation.
This allows devices to access the NET, even during Access Intervals
where SYNC is skipped, by using an Implicit Idle Sense algorithm.
If the hopping sequence is 79 frequencies in length as shown in
FIGS. 3a and 3b, use of every third Access Interval guarantees that
a SYNC message will be transmitted on each frequency within the
hopping sequence once each three cycles of the sequence, regardless
of whether 1, 2 or 4 Access Intervals occur each hop dwell. This
addresses U.S. and European regulatory requirements for uniform
channel occupancy, and improves the prospects for synchronization
of new units coming into the NET during periods when the NET is
otherwise inactive. SYNC messages that are on multiples of 3 Access
intervals are labeled as priority SYNC messages. "Sleeping"
terminals use priority SYNCs to manage their internal sleep
algorithms. Sleeping terminals and Implicit Idle Sense are
discussed in more detail below.
It should be noted that SYNC messages are preceded by dead time,
which must be allocated to account for timing uncertainty between
NET clocks and local clocks within NET constituents. In frequency
hopping systems, the dead time must also include frequency
switching time for the RF modem.
The Reservation Poll frame 203 immediately follows the SYNC header
201. The two messages are concatenated High-Level Data Link Control
frames separated by one or more Flags. The reservation poll
provides NET constituents an opportunity to gain access to the
media. It includes:
1. A field specifying one or more access slots.
2. A field specifying a probability factor between 0 and 1.
3. A list of addresses for which the base stations has pending
messages in queue.
4. Allocation of Time Division Multiple Access slots for scheduled
services by address.
5. Control Point device Transmitted Power level for SYNC and
Reservation Polls.
The number of access slots, n, and the access probability factor,
p, are used by the Control Point device to manage contention on the
channel. They may each be increased or decreased from Access
Interval to Access Interval to optimize access opportunity versus
overhead.
If the NET is lightly loaded, the pending message list is short,
and the NET is not subject to significant interference from other
nearby NETs, the control point device will generally specify a
single slot 501 as shown in FIG. 5a, with a p factor <1. In this
case, the reservation phase is Idle Sense Multiple Access ("ISMA").
Devices with transmission requirements that successfully detect the
Reservation Poll will transmit a Request for Poll ("RFP") with
probability p and defer transmission with probability 1-p. FIG. 5b
shows a device response (address 65 503 following the reservation
poll.
In cases when the transmission density is higher, n multiple
reservation slots will be specified, generally with a probability
factor p of 1. In this case a device will randomly choose one of n
slots for transmission of their Request for Poll. The slotted
reservation approach is particularly appropriate in instances where
many NETs are operating in near proximity, since it diminishes
reliance on listen before talk ("LBT") (explained more fully
below). The number of slots n is determined by a slot allocation
algorithm that allocates additional slots as system loading
increases. FIG. 6a shows multiple slots 601.
In cases where NET loading is extreme, the Control Point may
indicate a number of slots, e.g., not more than 6, and a
probability less than 1. This will cause some number of devices to
defer responding with a Request for Poll in any of the slots. This
prevents the control point device from introducing the overhead of
a large number of slots in response to heavy demand for
communications, by dictating that some units back off until demand
diminishes.
A pending message list is included in the Reservation Poll. The
pending message list includes the addresses of devices for which
the Control Point device has messages in queue. Devices receiving
their address may contend for the channel by responding with a
Request For Poll (RFP) in the slot response phase. FIG. 6b shows
several devices 603, 605 and 607 contending for channel access.
Messages that the Control Point device receives through the wired
infrastructure that are destined for Type 1 devices, and inactive
Type 3 devices whose awake window has expired, are immediately
buffered, and the device addresses are added to the pending message
list. When a message is received through the infrastructure for a
Type 2 device, or an active Type 3 device, their address is
prioritized at the top of the polling queue. (Device Types and
polling queue are described below.) The pending message list is
aged over a period of several seconds. If pending messages are not
accessed within this period, they are dropped.
Devices with transmission requirements respond in slots with a
Request for Poll. This message type includes the addresses of the
Control Point device and requesting device, the type and length of
the message it has to transmit, and a field that identifies the
type of device. Devices that detect their address in the pending
message list also contend for access in this maimer.
As mentioned above, devices may be Type 1, Type 2, or Type 3. Type
1 devices are those which require critical battery management.
These may be in a power saving, non-operational mode much of the
time, only occasionally "waking" to receive sufficient numbers of
SYNC and Reservation Poll messages to maintain connectivity to the
NET. Type 2 devices are those that are typically powered up and
monitoring the NET at all times. Type 3 units are devices that will
remain awake for a window period following their last transmission
in anticipation of a response. Other device types employing
different power management schemes may be added.
Slot responses are subject to collision in both the single and
multiple slot cases. Collisions may occur when two or more devices
attempt to send Request for Polls in the same slot. However, if the
signal strength of one device is significantly stronger than the
others, it is likely to capture the slot, and be serviced as if it
were the only responding unit. FIG. 6b shows two devices 605,
address 111, and 607, address 02, that may be subject to collision
or capture.
The Control Point device may or may not be able to detect
collisions by detecting evidence of recovered clock or data in a
slot, or by detecting an increase in RF energy in the receiver
(using the Received Signal Strength Indicator, ("RSSI"))
corresponding to the slot interval. Collision detection is used in
the slot allocation algorithm for determining addition or deletion
of slots in upcoming Reservation Polls.
As an optional feature to improve collision detection in the
multiple slot case, devices that respond in later slots may
transmit the addresses of devices they detect in earlier slots as
part of their Request for Poll. Request for Polls which result in
collisions at the Control Point device often are captured at other
remote devices, since the spatial relationship between devices that
created the collision at the base does not exist for other device
locations within the NET. The duration of the response slots must
be increased slightly to provide this capability.
If the Control Point device receives one or more valid Request for
Polls following a Reservation Poll, it issues a Reservation
Resolution ("RR") Poll and places the addresses of the identified
devices in a polling queue. The Reservation Resolution message also
serves as a poll of the first unit in the queue. Addresses from
previous Access Intervals and addresses of intended recipients of
outbound messages are also in the queue.
If the Polling Queue is empty, then no valid Request for Polls were
received or collision detected and no Reservation Resolution poll
is issued. If within this scenario a collision is detected, a CLEAR
message indicating an Explicit Idle Sense (explained more fully
below) is transmitted containing a reduced probability factor to
allow colliding units to immediately reattempt NET access.
Outbound messages obtained through the network infrastructure may
result in recipient addresses being prioritized in the queue, that
is, if the recipients are active devices--Type 2 devices or Type 3
devices whose awake window has not expired. This eliminates the
need for channel contention for many outbound messages, improving
efficiency. Messages for Type 1 devices are buffered, and the
recipient address is placed in the pending message list for the
next Access Interval.
Generally the queue is polled on a first in first out (FIFO) basis.
The polling order is:
a. Addresses of active units with outbound messages.
b. Addresses from previous Access Intervals
c. Addresses from the current Access Interval
Since propagation characteristics vary with time and operating
frequency, it is counterproductive to attempt retries if Poll
responses are not received. If a response to a Poll is not
received, the next address in the queue is polled after a short
response time-out period. Addresses of unsuccessful Polls remain in
the queue for Polling during the next Access Interval. Addresses
are aged, so that after several unsuccessful Polls they are dropped
from the queue. Addresses linked to outbound messages are added to
the pending message list. Devices with inbound requirements must
re-enter the queue through the next reservation phase.
Data is transferred in fragments. A maximum fragment payload of 256
bytes is used in the preferred implementation. If transfer of
network packets larger than of 256 bytes is required, two or more
fragments are transferred. Fragments may be any length up to the
maximum, eliminating the inefficiency that results when messages
that are not integer multiples of the fragment length are
transmitted in systems that employ fixed sizes.
The sequence for transferring data from a remote device to the
control point device is illustrated in FIG. 7a. It is assumed that
address 65 is the first address in the polling queue. The
Reservation Resolution poll 701 from the control point device
includes the device address and the message length that device 65
provided in its initial Request for Poll. A first fragment 703
transmitted back from device 65 is a full length fragment. Its
header includes a fragment identifier and a field providing
indication of the total length of the message. Length information
is included in most message types during the sessions period to
provide reservation information to devices that may wish to attempt
to access the NET following an Explicit Idle Sense (explained more
fully below).
Following successful receipt of the first fragment, the Control
Point device sends a second poll 705, which both acknowledges the
first fragment, and initiates transmission of the second. The
length parameter is decremented to reflect that the time required
for completion of the message transfer is reduced. A second
fragment 707 is transmitted in response, and also contains a
decremented length field. Following receipt of the second fragment
707, the Control Point device sends a third poll 709. This pattern
is continued until a final fragment 711 containing an End of Data
(EOD) indication is received. In FIG. 7, the final fragment is
shorter than a maximum length fragment. The Control Point device
sends a final Acknowledge (ACK), and the device sends a final CLEAR
713 to indicate conclusion of the transmission. The CLEAR message
contains a probability factor p for Explicit Idle Sense (explained
more fully below). The value of p is determined by the Control
Point device in the ACK and echoed by the device termination
communication. A p of zero indicates that the control point device
will be initiating other communications immediately following
receipt of the CLEAR message. A probability other than 0 indicates
an Explicit Idle Sense.
If for some reason a fragment is not successfully received, the
next poll from the Control Point device would indicate a REJECT,
and request re-transmission of the same fragment. The length field
would remain fixed at the previous value, prolonging reservation of
the channel for the duration of the message. After a fragment is
transmitted more than once without successful reception, the
Control Point device may suspend attempts to communicate with the
device based upon a retry limit, and begin polling of the next
address in the queue.
A flow chart depicting how inbound messages are received during an
access interval is shown in FIGS. 19A and 19B. A flow chart
depicting how outbound messages are transmitted during an access
interval is shown in FIGS. 20A and 20B.
Outbound messages are transmitted in a similar fashion as inbound
messages, with the Control Point and device roles largely reversed
as illustrated in FIG. 7b. When the Control Point reaches an
address in the queue for which it has an outbound message, the
Control Point transmits a Request for Poll 721 identifying the
address of the device and the length of the message. The response
back from the device would be a poll with an embedded length field.
The same POLL/FRAGMENT/ACK/CLEAR structure and retry mechanisms as
described above with regard to inbound messages in reference to
FIG. 7a are maintained. The CLEAR from the device indicates a
probability p of zero. If the polling queue is empty, the Control
Point may send a final or terminating CLEAR 723 containing a
probability for Explicit Idle Sense.
All terminating ACK or CLEAR messages contain fields to aid in
synchronization of new units to the NET. The content of these
fields is identical to that in the SYNC message, except that the
tinting character is deleted. Synchronization is discussed more
fully below.
Broadcast Messages intended for groups of addresses, or all
addresses within a NET may be transmitted during the sessions
period. Broadcast messages are not individually acknowledged. These
messages may be communicated at intervals over the course of
several Access Intervals to provide reliable communication.
Messages such as SYNC and Reservation Polls are specialized
broadcast messages, with dedicated bandwidth in the Access Interval
structure.
Security of payload data is left to the higher protocol layers.
Application programs resident in portable/mobile devices may employ
encryption or other means of providing protection against undesired
use of transmitted data.
Portable/mobile devices may employ transmitter power control during
the sessions period to reduce potential interference with other
NETs that may occasionally be on the same or adjacent channels.
These devices will use Received Signal Strength Indicator readings
from outbound messages to determine if transmitter power may be
reduced for their inbound transmission. Because of the need to
maintain channel reservations and Listen Before Talk capabilities,
the Control Point device does not use transmitter power control.
Since Control Point devices are generally part of an installed
system infrastructure, they are likely to be physically separated
from devices operating in other NETs. They are therefore less
likely to cause interference to devices in other NETs than portable
devices, which may operate in proximity to devices in other
NETs.
Often, control point devices will empty the polling queue before
the conclusion of the access interval. Two mechanisms within the
Access Control Protocol, Explicit and Implicit Idle Sense, are
provided to improve bandwidth utilization. These supplemental
access mechanisms often provide means for devices that failed to
gain reservations during the reservation phase to gain access to
the NET within the Access Interval. To assume an Explicit or
Implicit Idle Sense, a device must have detected a valid SYNC and
Reservation Poll in the current Access Interval.
The incorporation of a probability factor p.noteq.0 in the final
(terminating) ACK or CLEAR from the control point device provides
the function of an Explicit Idle Sense (mentioned above). Devices
with transmission requirements solicit Request for Polls using the
same rules normally used for a single slot Reservation Poll.
Successfully identified addresses are placed in the polling queue,
and are polled immediately or in the next Access Interval depending
on the time remaining in the current Access Interval. The p factor
for Explicit Idle Sense is subject to the same optimization
algorithm as the Reservation Poll probability.
Communication of channel reservations, in the form of the length
fields in Polls and Message Fragments is useful to units seeking to
access the NET through Explicit Idle Sense. Reservations allow
devices to predictably power down during the period that another
device has reserved the NET to conserve battery power, without
loosing the ability to gain access to the NET.
Implicit Idle Sense provides an additional means of channel access.
An Implicit Idle Sense is assumed whenever a device detects a quiet
interval period greater than or equal to the duration of a Poll
plus the maximum fragment length after a channel reservation has
expired. Detection based upon simple physical metrics, such as a
change in Received Signal Strength Indicator or lack of receiver
clock recovery during the quiet interval, are preferred methods of
ascertaining channel activity. Algorithms based upon these types of
indicators are generally less likely to provide a false indication
of an inactive channel than those that require successful decoding
of transmissions to determine channel activity. False invocation of
an Implicit Idle Sense is the only mechanism by which data
transmissions are subject to collision within the NET. Thus, the
Implicit Algorithm must be conservative.
Quiet interval sensing may begin at the following times within the
Access Interval:
a. Any time after the last reservation slot following a Reservation
Poll;
b. Any time after a terminating ACK or CLEAR indicating an Explicit
Idle Sense;
c. Following an unsuccessful response to a single Slot Reservation
Poll; or
d. Any time prior to reserved Time Division Multiple Access time
slots at the end of the Access Interval.
It is preferable that devices detecting a quiet interval use a p
persistent algorithm for channel access to avoid collisions. The
probability factor for Implicit Idle Sense Access will generally be
less than or equal to the factor in Explicit Idle Sense.
A device must receive the SYNC and Reservation Polls at the
beginning of an Access Interval to use Implicit Idle Sense. The
Reservation Poll provides indication of guaranteed bandwidth
allocation to scheduled services at the end of the Access Interval,
which may shorten the period available for Bandwidth On Demand
communications.
Devices requiring scheduled services must contend for the channel
in the same fashion as those requiring Bandwidth On Demand access.
When polled, these initiating devices will initiate a connection
request that indicates the number of inbound and outbound Time
Division Multiple Access slots required for communication, and the
address of the target device with which communication is desired.
The network infrastructure will then attempt to establish the
connection to the target device. Once the connection is
established, the Control Point device will signal the allocation of
slots to the initiating device. Time Division Multiple Access slots
are relinquished by transmitting a disconnect message to the
control point device in the Time Division Multiple Access slot
until the disconnect is confirmed in the next Reservation Poll.
The transmission requirements of speech and slow scan video
(scheduled services) are similar. In one embodiment, Time Division
Multiple Access slots are allocated as multiples of 160 bits
payload at 1 MBIT/sec, plus overhead for a total of 300 .mu.s. For
10 ms access intervals, acceptable voice communication can be
obtained by allocating 1 Time Division Multiple Access slot each
for inbound and outbound communication per access interval. For 20
ms access intervals, two slots each way are required. A system
employing 10 ms access intervals at 100 hops per second may improve
transmission quality by using two or three slots each Access
Interval and sending information redundantly over two or three
access intervals using interleaved block codes. Scheduled
transmissions are generally not subject to processing or validation
by the control point device, and are passed through from source to
destination. Use of interleaved error correction coding or other
measures to improve reliability are transparent to the NET.
The selection of certain system parameters are important when
considering scheduled services. As an example, since speech is
quantized over the duration of the access interval and transmitted
as a burst, the length of the access interval translates directly
into a transport delay perceptible to the recipient of that speech.
In real time voice communications, delays longer than 20 ms are
perceptible, and delays longer than 30 ms may be unacceptable. This
is particularly the case where the premises LAN is interconnected
with the public switched telephone network ("PSTN"), which
introduces its own delays. Two way services such as voice
communications are the most sensitive to transport delay because
delay impacts the interaction of the communicating parties. One way
services are less sensitive to transport delay. One way services
are good candidates for interleaving or other forms of redundant
transmission.
Similarly, the selection of hop rate is important, as hop rate
determines the duration of outages that may occur. If one or more
frequencies in the hop sequence are subject to interference, for
instance, scheduled transmissions during those hops will be
disrupted. In a system that hops slowly, detrimental outages of
hundreds of milliseconds will occur resulting in poor transmission
quality. Occasional losses of smaller durations, e.g., 10 ms or 20
ms, are generally less perceptible, indicating that faster hop
rates are desirable if the NET is to offer real time voice
transport.
Scheduled service intervals may also be used for data transport on
a scheduled or priority basis. Telemetry, data logging, print
spooling, modem replacement, or other functions are possible. For
these activities, a few Time Division Multiple Access slots
scheduled for example every fourth, eighth, or sixteenth Al are
necessary.
Because of multipath and dispersion issues with 2.4 GHz
transmission at relatively high data rates, the ability of the NET
to adaptively switch between two or more data rates is
desirable.
In one embodiment, implementation of data rate switching may be
accomplished by selecting a standard rate of communications, e.g.,
250 KBPS and high rate of communications of 1 Mbit/sec. Messages
that contain system status information, including SYNC, Reservation
Polls, Reservation Resolution Polls (Request for Polls), Polls,
ACKs and CLEARS are transmitted at the standard rate. These
messages are generally short, and the time required for
transmission is largely determined by hardware overhead, e.g.,
transmitter receiver switching time. The incremental overhead
introduced by transmitting these messages at the lower rate is
therefore small in comparison to the total length of an access
interval. The reliability of reception of these messages will
increase, which will eliminate unnecessary retries in some
instances where fragments are received successfully, but
acknowledgements or polls are missed.
A test pattern at the higher data rate is inserted in each Poll
(not in Reservation Polls, however). The Poll recipient evaluates
signal quality based on the high data rate test pattern, Received
Signal Strength Indicator, and other parameters to determine
whether to transmit a fragment at the high rate or the low rate.
Fragment lengths are selected such that high and low rate maximum
fragment lengths are the same duration. In other words, a fragment
at the low rate conveys approximately 1/4 the payload of a fragment
for the case where the data rate is four time greater. This method
is generally suitable for transaction oriented communications,
which frequently require short message transmissions.
Alternatively, the length field in Polls and messages can be used
to allow different fragment lengths for the two data rates while
still providing channel reservation information to other devices in
the NET. This method also provides for forward migration. As
modulation and demodulation methods improve, newer products can be
added to old networks by upgrading Control Points devices. Both new
and old devices share the ability to communicate at a common low
data rate.
An alternate embodiment uses signaling messages such as SYNC,
Reservation Polls, Request for Polls, etc., at the higher rate with
fallback operation to the standard rate for the communications
sessions only. SYNC and Reservation Polls at the high rate
constitute a high data rate test message. The Request for Poll
response to the Reservation Poll at the high rate may include a
field indicating that sessions communications should take place at
the fallback, standard rate. Signal quality measures such as signal
strength and clock jitter are appropriate. Data rate selection
information is included with the device address in the polling
queue. When the device is polled, it will be polled at the rate
indicated in the Request for Poll. Channel reservation information
in the Reservation Resolution Poll will indicate the reservation
duration based upon the data rate indicated.
In this alternate embodiment, the fact that SYNC and Reservation
Polls must be detectable at the high data rate prioritizes access
to the NET for those devices that have acceptable connectivity
during the current access interval. This general approach has
desirable characteristics in a frequency hopping system, as the
propagation characteristics between devices may change
significantly as the NET changes from frequency to frequency within
the hopping sequence, or over several Access intervals during the
dwell time on a single frequency. Reduction in data rate in this
system is primarily intended to remedy the data smearing
(inter-symbol interference) effects of dispersion due to excess
delay, rather than temporary poor signal to noise ratio due to
frequency selective fading. Devices that receive high data rate
transmissions with acceptable signal strength but high jitter are
likely to be experiencing the effect of dispersion.
The concept of allowing Polls and message fragments to occur at
either a high or low data rate could create difficulties for other
NET constituents that need to be able to monitor the channel for
reservation information. Two embodiments for solving this problem
are the use of auto-discriminating receivers or the use of fixed
data rate headers for system communications.
Auto discrimination requires the receiver to process messages sent
at either data rate, without necessarily having prior knowledge of
the rate.
Given a high rate of 1 MBIT/SEC, and a low Rate of 250 KBPS, i.e.,
one being a binary multiple of the other, it is possible to devise
preambles that can be received at either rate. Consider that 01 and
110 sent at the low rate correspond to 00001111 and 111111110000 at
the high rate. These preambles are transmitted continuously before
the transmission of the High-Level Data Link Control FLAG character
at the correct data rate indicating the start of a message. In this
example, a preamble of 20 bits of 01 at the low rate indicates
operation at the high rate. A preamble of 30 bits of 110 indicates
operation at the low rate. A receiver tuned to either rate is
capable of receiving both types of preambles and initiating the
proper decoding mechanisms for the intended rate of
transmission.
This general technique, with appropriate selection of preamble
content, is applicable to binary modulation schemes, for example, a
frequency modulated system where a common frequency deviation value
is used for both data rates. It is also applicable to systems where
switching occurs between binary and multilevel modulation, such as
disclosed in U.S. patent application Ser. No. 07/910,865, filed
Jul. 6, 1992.
Referring now to FIG. 25, a preamble 2501, a SYNC 2503 and a
Reservation Poll 2505 is illustrated. The preamble 2501 starts at
the beginning of the Access Interval 2500 and is applied to an RF
modem while it is switching frequencies. Since the switching time
is a worst case, this causes the preamble 2501 to be present and
detectable prior to the allocated 150 .mu.sec period in some
instances. It would be equally appropriate to begin preamble
transmission 50 or 100 .mu.sec into the switching period if that
would be more convenient. The timing has been selected to allow 100
.mu.sec.
Referring to FIG. 26, a sample SYNC message 2600 is shown.
Referring to FIG. 27, a sample Reservation Poll 2700 is shown. In
these examples, the hopping synchronization information has been
positioned in the Reservation Poll 2700.
With auto-discrimination, it is possible to change data rates on a
per-poll basis, thereby adjusting for channel temporal dynamics.
Since all devices in the NET have auto discrimination capabilities,
and channel reservation information is included in message headers
as a length field, the bandwidth reservation features of the NET
are preserved. The maximum fragment duration may be maintained at a
fixed value, meaning that low data rate fragments convey less data
than their high rate counterparts, or may be scaled in the ratio of
the data rates to allow consistent fragment data payloads.
An alternative to auto-discrimination is the use of headers to
communicate system information. This embodiment is less preferred,
but may be appropriate if economics, size, or power constraints
dictate a simpler design than that required for
auto-discrimination. In this embodiment, any transmission at the
lower data rate is preceded by a header at the high data rate that
conveys NET management information, i.e., channel reservation
status. Devices other than those directly involved in polling or
fragment transmission need only monitor at the high rate for
channel reservation information. The header at the high rate and
the following transmission at the low rate are concatenated
High-Level Data Link Control frames, with an appropriate preamble
for low rate clock recovery synchronization in-between.
For the communicating devices, the header can serve the additional
purpose of acting as a test pattern at the high rate. For example,
if a device is polled at the low rate, but successfully decodes the
high rate header with adequate signal quality, it may indicate back
to the polling unit to poll again at the high rate.
In a premises LAN as discussed in reference to FIG. 1, many NETs
may be distributed geographically to provide enhanced coverage or
additional system capacity. The wired portion of the network
infrastructure, such as Ethernet or Token Ring, provides a means
for coordination of NETs to achieve optimum system performance. An
equally important role of the wired infrastructure is to allow
resource sharing. Portable devices with limited memory capacities,
processing power, and relatively small batteries may access large
data bases on, or remotely initiate processing capabilities of,
larger AC powered computer systems. Portable/mobile devices may
also share communication with other like devices which are serviced
by other NETs well beyond the radio coverage range of their own
NET.
The basic method for communication of status information regarding
the premises LAN is the HELLO message. HELLO messages are sent
routinely, but relatively infrequently, for example, every 90
Access Intervals. The HELLO transmission interval is tied to the
Priority SYNC interval, so that the HELLO interval corresponds to
Access Intervals where SYNC is transmitted if the network is
lightly utilized.
In an alternate embodiment, HELLOs could be inserted as a broadcast
message at the beginning of the Sessions period. FIG. 8 illustrates
a preferred Access Interval embodiment where a HELLO message 801 is
inserted between a SYNC 803 and a Reservation Poll 805. The SYNC
frame at the beginning of the Access Interval indicates that the
Access Interval will contain a HELLO, allowing power managed
devices to remain awake to receive the HELLO.
HELLO messages may also contain information regarding pending
changes in the local NET. If the local NET is changing Access
Interval durations or hop sequences, for instance, changes may be
communicated in several consecutive HELLOs so that the information
is reliably communicated to all NET constituents, permitting all
devices to make the change in coordinated fashion. Further
discussion of HELLO message content is provided below.
For purposes of channel management in the Access Interval
structure, the maximum transmission duration by a device should be
limited to the time that the device moving at a maximum expected
velocity can traverse 1/4 wavelength of the maximum carrier
frequency. The duration may be further reduced to compensate for
link bit error rate characteristics or expected duration or
frequency of interference bursts. A maximum transmission duration
of 2.5 ms is suitable for 1 MBIT/SEC transmission, with a device
velocity of 15 mph, in a multiple NET environment.
Use of spatial or polarization antenna selection diversity is also
desirable in indoor propagation environments. First, the receiving
unit makes an antenna diversity decision during the preamble
portion of each transmission. The antenna used for reception for
each device address is then recorded in memory so that the correct
antenna will be used for response messages to each address. While
diversity selection is only valid for a short time, it is not
necessary to age this information, because antenna selection is
equi-probable even after diversity information is no longer
valid.
The Access Interval structure also inherently provides routine
channel sounding for each hop. This is important in a frequency
hopping system, as channel conditions will vary considerably from
frequency to frequency within the hopping sequence. NET
constituents must, in most cases, be able to receive SYNC and
Reservation Poll transmissions from the Control Point device to
attempt inbound access in an Access Interval. This provides a
positive indication that the device is not experiencing a channel
outage, allowing power saving and eliminating possible channel
contention. Channel sounding does not need to be employed during
periods where the NET is not busy since contention is unlikely in
this situation.
Channel sounding for Outbound messages is accomplished through a
Request for Poll/Poll cycle where handshaking messages with short
time out periods must be successfully communicated before longer
message transmissions may be attempted.
As discussed above with regard to FIG. 1, a premises LAN consists
of several base stations 15 located throughout an environment
requiring wireless communications, e.g., a building or other
facility, or a campus comprised of several buildings. The base
stations 15 are placed to provide coverage of intended usage areas
for the roaming portable or mobile computing devices 20. Coverage
areas must overlap to eliminate dead spots between coverage
areas.
The base stations 15 may be interconnected via industry standard
wired LANs, such as IEEE 802.3 Ethernet, or IEEE 802.5 Token Ring.
Base stations may be added to an existing LAN without the need to
install additional LAN cable. Alternatively, it may be desirable to
install base stations on dedicated LAN segments to maximize
performance of both the radio network and other collocated computer
devices.
Base stations within the premises LAN provide Control Point
functions for individual NETs. NETs employ different hopping
sequences to minimize potential interference between NETs.
Regulatory restrictions generally preclude synchronization of
multiple NETs to a single master clock, requiring that individual
NETs operate independently from one another. The lack of the
ability to coordinate timing or frequency usage between NETs
introduces the potential for collisions between independent NETs
with overlapping coverage areas.
FIGS. 9a and 9b illustrate conceptually how multiple NETs may be
employed in an idealized "cellular" type installation. Each hexagon
901 and 903 in FIG. 9a represents the primary coverage area of a
given NET. Coverage areas are modeled as circles 905 based upon
some reliability criterion, for example a 5% mean fragment retry
rate (on average 95% of fragments are successfully communicated on
the first attempt). Typical coverage areas are determined by
physical attributes of the area in which the NET operates. As is
illustrated in FIG. 9b for the hexagon (NET) 903 of FIG. 9a, an
actual coverage area 907 meeting the reliability criterion is
likely to be irregular. This may require base stations to be offset
significantly from the hexagonal grid.
FIG. 10 illustrates a coverage contour overlap for the multiple
NETs in the premises LAN of FIG. 1. Dark shaded areas 1001 indicate
areas where base station coverage overlaps. Because the coverage
distance of a radio system on an instantaneous basis greatly
exceeds the coverage that can be provided on average to sustain a
given quality of service, the overlap at any instant may be
significantly greater than the coverage contours indicate.
FIG. 11 illustrates hopping sequence reuse in a multiple NET
configuration. Hopping sequence re-use may be necessary if there
are physical constraints on the number of hopping sequences that
can be supported. For example, devices may have limited memory
available for hopping sequence storage. Use of a smaller set of
sequences also simplifies the task of determining sets of sequences
that have acceptable cross correlation properties. In FIG. 12, 7
hopping sequences 1 through 7 are used throughout the coverage
area. Other NETs may reuse the same hopping sequence at some
distance removed. While 7 NETs are illustrated, larger numbers,
such as 9 or 15 may provide a better compromise between minimizing
the number of hopping sequences used, and reuse distance between
NETs using the same sequence. Reuse requires coordination of
hopping sequence assignment--either the system installer can
coordinate the installation, or the system may include automated
management features to assign hopping sequences to individual
NETs.
Since NETs are not synchronized, different NETs that use the same
hopping sequence are likely to interfere during periods where
oscillator drift causes them to be temporarily synchronized. At
other times, they may only interfere due to imperfect
channelization. For example, for a worst case 100 ppm frequency
error between two NETs using the same 79 frequency sequence at one
Access Interval per hop and 50 hops per second, NETs will partially
or fully overlap for a duration of 10 minutes every 4.3 hours.
Typically the frequency error will be 25% to 50% of the worst case,
leading to longer overlap periods occurring less frequently.
NETs using the same hopping sequence must be physically isolated
from one another to reduce interference to an acceptable level.
Extensive hopping sequence reuse generally requires site
engineering and optimization of base station placement. Using more
hopping sequences reduces the need for critical system engineering
during installation. Fifteen hopping sequences is a preferred
number for hopping sequence reuse, allowing simplified installation
and minimal coordination.
NETs that use different hopping sequences will also temporarily
synchronize in timing relationships that cause mutual co-channel
interference on common channel frequencies. Since the number of
channels that must be used in a sequence is a significant fraction
of the total number of channels available, all sequences will share
some number of frequencies in common. When sequences are time
aligned so that a common frequency is used simultaneously,
interference can occur. Optimization of sets of sequences for low
cross correlation is necessary to prevent various time alignments
of sequences from having more than one or two frequencies in
common.
Optimization of hopping sequences for multiple NETs must also
include analysis of imperfect channelization. The performance
characteristics of the RF modems may not, for economic or power
consumption reasons, provide sufficient transmitter spectral
containment, receiver dynamic range, or receiver selectivity to
guarantee that devices operating on different frequencies in
proximity to one another will not interfere. In selecting hopping
sequences for desirable cross correlation properties, adjacent and
alternate adjacent channel interference must be considered.
Protocol retry mechanisms for fragments lost to adjacent channel
interference or limited dynamic range may be randomized to prevent
continued disruption of communications in the affected NET.
Often in campus environments where systems must provide coverage in
several buildings, the cost of wiring LAN cable between base
stations is prohibitive. To establish connectivity between base
stations in an premises LAN, it may be necessary to provide
wireless links between groups of base stations connected to
separate LAN segments. FIG. 12 illustrates a wireless link 1201
connecting groups of base stations 1203 and 1205. The base stations
1203 and 1205 are connected on separate LAN segments 1207 and
1209.
In one embodiment, the base stations 1203 and 1205 may be
configured in a wireless point to point mode, wherein one base
station serves as a control point device while the others operate
in a slaved mode dedicated to point to point data transfer. Slave
base stations are configured to operate as portable/mobile devices,
and forward communications to master bases by sending Request for
Polls during reservation opportunities or Implicit Idle Sense
periods. Because of the potential high traffic of point to point
links, separate NETs may be allocated for this purpose, with a
master communicating with one or more slave units. Master units may
also communicate with other portable/mobile devices. The COST
weighing (discussed below) in a slave's HELLO transmission is
preferably set to a high value, to force portable/mobile devices
which can connect to another NET to do so.
In another embodiment, it may also be desirable to support wireless
base stations. Wireless base stations serve as control points, but
are not connected to the infrastructure through a LAN cable. As is
illustrated in FIG. 13, a wireless base station 1301 participates
in the premises LAN through a wireless link 1303 to a base station
1305 that is connected to a LAN 1307.
Wireless base stations operate as slave devices to master base
stations which are connected to the wired infrastructure. The wired
and wireless base stations share the same hopping sequence, and are
synchronized as a common NET. Because they are not connected to the
Infrastructure, wireless base stations must be used as store and
forward devices. Each transmission to a wireless base must be
retransmitted to the intended destination device, doubling the
number of transmissions occurring in the NET. Wireless base
stations are preferably used for supplementing coverage area of the
premises LAN. For example, a wireless base station might provide
spot coverage of isolated "dead spots" where data traffic is
limited or where providing a wired LAN connection is difficult.
Wireless base stations may also serve as emergency spares to
provide coverage in the event of a failure of a primary base
station. In this role, the wireless base station may be either
permanently installed in selected locations, or stored in a
maintenance area and quickly positioned and connected to AC or
battery power to provide communications while repairs are made to
the primary wired base station. Moreover, permanently installed
wireless base stations might also be used for redundancy, i.e., to
monitor an associated base station and to take over when a
breakdown is detected.
The preferred wireless base station embodiment uses interleaved
access intervals. The parent wired base station and secondary
wireless base station coordinate Access Intervals, the wired base
station deferring every third or sixth access interval to the
wireless base. Since the wired base station transmits priority SYNC
messages every third Access Interval, the wireless base station may
routinely be allocated one of the two intervening Access Intervals
for priority SYNC communications with devices that are attached to
it. Communication between the wired and wireless base stations may
occur during Access Intervals initiated by either base station.
Wireless base stations may also communicate with devices during an
Access Interval using Implicit or Explicit Idle Sense.
This embodiment provides predictable access for devices attached to
the wireless NET, and allows the same power management algorithms
to be used regardless of whether the base station is wired or
wireless. The wireless base station may transmit its own priority
SYNC and HELLO messages. Also, devices seeking communications with
the wireless base station will automatically be synchronized with
the wired base as well, allowing immediate improved access to the
network if their mobility has put them within range of the wired
base.
Because of the constraint of sharing bandwidth with a wired base
station, connectivity of wireless base stations is normally limited
to one per wired base station. However, in cases where system
loading is predictably and consistently light, multiple wireless
base stations could share a single wired base, e.g., each
transmitting in turn in the Access Intervals between the Wired Base
Priority SYNC Access Intervals.
Wireless base stations are capable of supporting scheduled traffic.
However, since each transmission to a wireless base station must be
forwarded, scheduled transmissions through wireless base stations
use twice the bandwidth as those through wired base stations. In
other words, twice the number of Time Division Multiple Access
slots must be allocated. To avoid introducing excessive delay,
communications must be forwarded during the same Access Interval
that they are received, or shorter Access Intervals must be used.
Scheduled traffic slot assignments must be common to all wireless
bases operating within a single NET.
Wireless base stations require reliable communication with their
wired counterparts. This dictates smaller coverage contours for
wireless base stations. If a wired base station provides 80,000
square feet of coverage area, a wireless base can be predicted to
provide only an additional forty percent coverage improvement, due
to overlap with the wired base station. Frequently, base stations
are mounted at ceiling level, providing a relatively clearer
transmission path between base stations than exists between bases
and portable/mobile devices located in more obstructed areas near
the floor. With careful site engineering and installation, a
wireless base station can provide somewhat better than the forty
percent predicted improvement, but still less than the coverage of
an additional wired base.
As discussed above, HELLO messages are used to communicate NET and
premises LAN status messages. They facilitate load leveling and
roaming within the premises LAN and allow sequence maintenance to
improve security and performance within the NET. HELLO messages
occur periodically in Access Intervals that contain priority SYNC
messages. HELLOs are sent periodically relative to the sequence
length, for instance, every 90 Access Intervals. HELLOs, like SYNC
information, are optionally encrypted to provide greater
security.
Each HELLO message includes a field for COST. COST is a measure of
the base station to handle additional traffic. A device determining
which of two or more base stations having adequate signal strength
to register which will select the base with the lowest COST
factor.
The base computes COST on the basis of how many devices are
attached to the NET, the degree of bandwidth utilization, whether
the base is wired or wireless, the number of frequencies
experiencing consistent interference within the sequence, and the
quality of the connection the base has within the premises LAN.
FIG. 14 illustrates the concept of base stations communicating
neighboring base station information through HELLO messages to
facilitate roaming of portable/mobile devices. In a premises LAN,
base stations 1401, 1403 and 1405 communicate SYNC information
amongst themselves via wired backbone (LAN) 1407. In addition, a
wireless base station 1409 (discussed above) similarly communicates
with the base stations 1401, 1403 and 1405 via a wireless link
1411. A portable/mobile device 1413 is initially registered with
base station 1401, which acts as a control point for the
portable/mobile device 1413. HELLO messages transmitted by base
station 1401 to portable/mobile device 1413 contain fields for
neighboring base stations 1403, 1405 and 1409. These fields may
indicate, for example, addresses of the neighboring bases, their
COST, the hopping sequences, hopping sequence indices, number of
Access Intervals per hop, and NET clock. The portable/mobile device
1413 detects the HELLOs transmitted from base station 1401 and uses
the information for coarse synchronization with the other base
stations 1403, 1405 and 1409. This permits the portable/mobile
device to roam between base station coverage areas (i.e., between
different NETs) without going through a full acquisition phase.
Roaming of portable/mobile devices is discussed in more detail
below.
Simply put, communication of neighbors' information permits each
base station to advise its associated portable/mobile devices
(i.e., those having common communication parameters) on how to
capture HELLO messages from neighboring base stations having
different communication parameters. Such communication parameters
may include, for example, hopping sequences, spreading codes, or
channel frequencies.
For example, neighbors' information transmission is appropriate in
any case where the system uses more than a single channel. For
instance, in a direct sequence architecture, a single spreading
code is often used. Capacity can be added to such a network by
employing different spreading codes at each base station. The
neighbors' information included in the HELLO message from a given
base station would include the spreading sequences of base stations
providing coverage in adjacent coverage areas. Likewise, in a
multiple frequency channelized system, HELLO messages would include
the channel frequencies of adjacent base stations.
In addition to facilitating roaming, communication of neighbors'
information may also facilitate the initial selection of a base
station by a portable/mobile device attaching to the premises LAN
for the first time.
Base station HELLO messages may also facilitate adaptive base
station transmitter power control. For example, each base station
HELLO transmission could specify the transmitter power level being
used by the base station. If a given attached portable/mobile
device notes that the current base station transmitter power level
is unnecessarily high (creating the possibility of interference
with other base stations), the portable/mobile unit could send a
message to the base station indicating as such, and the base
station could adjust the transmitter power level accordingly.
HELLO messages also enable communication of information indicating
to all devices that certain changes in the NET are required. For
example, the NET may switch hopping sequences periodically to
improve security, or to avoid interference sources that
consistently interfere with one or two frequencies within a given
sequence. Interference may result from outside sources, or from
other NETs. Changes to the NET are communicated over the course of
several HELLO messages (with a countdown) before the change occurs,
so that all devices are likely to be aware of changes and
synchronize at the instant of change.
In addition, if encryption is used, the encryption key may be
periodically changed in HELLOs. Like hopping sequence changes, KEY
changes are sent over several HELLOs, and are encrypted using the
existing key until the change goes into effect.
As mentioned above, roaming portable and mobile computing devices
operating in the premises LAN will routinely move between base
station coverage areas. At the maximum device velocity and expected
coverage area per base station, a mobile device may be expected to
cross a NET coverage contour in several seconds. Because of the use
of multiple, non-synchronized frequency hopping NETs, it is more
difficult to provide for simple hand-off between base stations than
it would be in a system that used cellular techniques with a single
frequency per cell. The premises LAN makes special provisions for
roaming by transmitting coarse frequency hopping synchronization
information in HELLO messages.
The premises LAN uses a spanning tree algorithm to maintain current
information regarding the general location of mobile devices within
the network. When a device changes registration from one NET
Control Point to another, routing information is updated throughout
the infrastructure. Wired base stations may broadcast spanning tree
updates to attached wireless base stations.
In the premises LAN, roaming portable and mobile devices initially
select and register with a Base Station Control Point on the basis
of link quality, i.e., signal quality, signal strength and COST
information transmitted within HELLO messages. A device will remain
attached to a particular base station until the link quality
degrades below an acceptable level, then it will attempt to
determine if an alternative NET is available. Different device
operating scenarios dictate different roaming strategies, discussed
below.
An idle device monitors SYNC and HELLO messages from the Control
Point device to maintain NET connectivity. Type 2 devices do not
employ power management, and always maintain their receivers in an
active state. They monitor all SYNC messages. Type 1 and Type 3
devices typically employ power management, operating in standby or
sleep modes of operation for many Access Intervals before
activating their receivers for monitoring SYNC and HELLO messages.
Control Points are guaranteed to send Priority SYNC frames every
third Access Interval. HELLOs occur every 30th Priority SYNC frame.
Power managed devices employ sleep algorithms synchronized to wake
for the minimum period necessary to guarantee receipt of priority
SYNC, HELLO, and Pending Message transmissions before resuming
SLEEP.
Type 2 devices are typically operated from high capacity vehicular
power systems, which eliminates the need for power management.
These devices may travel at velocities near the maximum system
design specification, dictating more frequent roaming. Type 2
devices will initiate a search for an alternative NET if SYNC
messages are consistently received at signal strengths below a
Roaming Threshold or if reception errors are consistently detected.
Because of the effects of frequency selective fading, signal
strength information is averaged over the course of several hops
within the hopping sequence.
If roaming is indicated, the device initiates a Roaming Algorithm,
using Neighbors' information from the most recent HELLO to attempt
synchronization with another candidate NET. If SYNC is not detected
within 6 hops, another candidate from the Neighbors list will be
selected, and the process repeated. Once SYNC is attained on an
alternative NET, the device will monitor signal strength and data
errors for several hops to determine link quality. If link quality
is acceptable, the device will continue monitoring until a HELLO is
received. If COST is acceptable, it will then register with the new
NET. The Control Point device will update the Spanning Tree over
the wired backbone (or by RF if a wireless base). If link quality
or COST is unacceptable, another candidate from the Neighbors list
is selected and the process repeated. This continues until an
acceptable connection is established. If a connection cannot be
established, the device must return to the original NET or employ
the initial acquisition algorithm.
Type 2 devices also have the option of monitoring other NETs before
degradation of their NET connection. They may do so by monitoring
their own NET for the SYNC and pending message list transmissions,
then scanning other candidate NETs during the Sessions period of
their NET. Other type devices may do so less frequently.
Type 1 and Type 3 devices may sleep extensively when idle,
preferably activating every nine Access Intervals to resynchronize
and check pending messages. Successful reception of at least one
SYNC during three monitoring periods is necessary to maintain fine
synchronization to the NET clock. Failure to receive two of three
SYNC frames, or receipt of two or three SYNC messages with poor
signal strength are possible indications of the need to further
test link quality by remaining active for several consecutive SYNC
transmissions. If signal strength or data errors over several hops
indicates that link quality is poor, or if a received HELLO message
indicates high COST, the roaming algorithm is initiated, and
alternative NETs are evaluated, as in the case of Type 2
devices.
Some battery powered devices may sleep for periods of time more
than nine Access Intervals. For example, devices with extremely
limited battery capacity may sleep between HELLOs, or several HELLO
periods, after which they must remain active for several
consecutive Access Intervals to regain fine synchronization and
assess whether to initiate roaming.
A Type 1, Type 2, or Type 3 device that has inbound message
requirements immediately activates its receiver and waits for a
SYNC and subsequent Reservation Opportunities. A device that does
not detect SYNC messages over the course of six Access Intervals
immediately initiates the Roaming Algorithm.
Outbound messages for devices that have changed coverage areas, but
which have not yet registered with a new Control Point device, are
problematic. For example, in the premises LAN, messages will be
forwarded to the Base Station that the device had previously been
attached to. The base station may attempt to poll the device during
one or more Access Intervals, then transmit the unit address in the
pending message list periodically for several seconds before
disregarding it. Once the unit attaches to a base, the message must
be transferred from the previous base station for delivery to the
unit. All of these activities require transmission bandwidth on
either the backbone or RF media, waste processing resources within
the premise LAN, and result in delayed delivery.
As this premises LAN embodiment is designed, the network has no
means of distinguishing messages it cannot deliver due to roaming
from messages that should be retried due to signal propagation
characteristics, interference, or sleeping devices. For this
reason, the roaming algorithm may be designed to allow devices to
quickly detect that they have lost connectivity within their
current NET, and re-attach to a more favorably located base
station.
Some improvement in delivering pending messages to roaming
terminals can be obtained by routinely propagating pending message
lists over the wired backbone. When a device attaches to a base
station, that base is able to immediately ascertain that the device
has a pending message, and initiate forwarding of the message for
delivery to the device.
In a preferred frequency hopping embodiment, the hopping sequence
consists of 3 m.+-.1 frequencies, where m is an integer. 79
frequencies are preferred. This embodiment will support hopping
rates of 100, 50 hops per second at 1 Access Interval per dwell, 25
hops per second at 2 frames per dwell, and 12.5 hops per second at
4 frames per dwell. Other rates can be supported for other Access
Interval Durations. For example, if the Access Interval is
optimized to 25 ms, hop rates of 80, 40, 20, and 10 hops per second
would be supported.
All devices within the NET may have one or more hopping tables that
contain potential hopping sequences that may be used. Up to 64
sequences may be stored in each device. Each sequence has an
identifier, and each frequency in each sequence has an index. The
sequence identifier and index are communicated in the SYNC
transmission.
All SYNC transmissions may be block encrypted to prevent
unauthorized devices from readily acquiring hopping synchronization
information. To facilitate encryption, the encryption key may
initially be factory set to a universal value in all devices. Users
would then have the option of changing this key, by providing a new
key to each device in the system. This may be accomplished through
keyboard entry or other secure means. Keys may also be changed
through the NET.
To facilitate hopping management, a hopping control portion of a
protocol controller will download a hopping table to a radio modem,
and will signal the radio modem when to hop. This approach
consolidates timing functions in the protocol controller, while not
requiring the controller to be concerned with conveying frequency
selection data to the modem each hop.
The NET may switch hopping sequences periodically to improve
security, or to avoid interference sources that consistently
interfere with one or two frequencies within a given sequence. As
mentioned above, changes to the NET are communicated over the
course of several HELLO messages before the change occurs so that
all devices are likely to be aware of changes.
Initial synchronization requires devices to ascertain the hopping
sequence, the hop rate, and the specific frequency from the hopping
sequence currently in use. Synchronization information is contained
in two types of routine messages. The SYNC field at the beginning
of an Access Interval contains synchronization information
including the hopping sequence, the index of the current frequency
within the sequence, the number of Access Intervals per hop, and
the length of the Access Interval. It also contains a timing
character that communicates the NET master clock to all listening
devices. Termination messages in the Sessions period, ACK and
CLEAR, contain the same information, but do not contain the timing
character.
The simplest method for attaining synchronization is to
Camp--select a quiet frequency that is likely to be within a
sequence in use--and listen for valid synchronization information.
If a SYNC message is detected, the listening device immediately has
both coarse and fine synchronization, and can begin the
registration process.
If SYNC is not detected, but a termination message is, then the
device has acquired coarse synchronization. The particulars of the
hopping sequence are known, but the boundaries of the dwells are
not. To acquire fine synchronization, it begins hopping at the
indicated hopping rate, listening for SYNC. If SYNC is not detected
after a reasonable number of hops, preferably 12 or 15, the device
reverts to camping.
The worst case scenario for synchronization is to synchronize to a
single NET that is idle. Given a 79 frequency hopping sequence, one
Access Interval per hop, and SYNC transmissions every third Access
Interval if the NET is idle, it may take nine cycle times to
guarantee that a SYNC transmission will be detected with 99.5%
probability. At 50 hops per second, synchronization could require
as long as 14 seconds. At 100 hops per second, 7 seconds is
required.
At 2 Access Intervals per hop, a SYNC transmission is guaranteed to
occur every frequency over 2 cycles of the hopping sequence. Six
cycles are required for 99.5% probability of acquisition,
corresponding to 19 seconds at 25 hops per second. At 4 Access
Intervals per hop, at least one SYNC is guaranteed to occur each
hop. Three cycles of the hopping sequence are required for 99.5%
acquisition probability. At 12.5 hops per second, this also
requires 19 seconds.
This illustrates the advantage of scalability. A device that uses
an acquisition algorithm suitable for 2 or 4 Access Intervals per
hop will also acquire a NET that hops at 1 Access Interval per hop.
The algorithm may be as follows:
1. The device scans candidate frequencies until it finds one with
no Received Signal Strength Indicator indication.
2. The device remains on the frequency for 6.32 seconds 2 Access
Interval/hop@25 Hops/second.times.2, or 4 Access Interval/hop@12.5
hops/second.times.1, or until it detects a SYNC message or a valid
termination message.
3. If SYNC is detected, the device synchronizes its internal clock
to the SYNC, and begins hopping with the NET for the next 11 hops.
It may attempt registration after detecting valid SYNC and any
Reservation Opportunity. If synchronization is not verified by
detection of SYNC within the 11 hops, the acquisition algorithm is
reinitialized.
4. If a message termination (either an ACK or CLEAR) is detected,
the device immediately hops to the next frequency in the sequence
and waits for the SYNC. It is coarsely synchronized to the NET but
has a timing offset from the NET clock.
When the next SYNC is received, the device synchronizes its clock
to the NET clock and initiates registration. If SYNC is not
received within a dwell time, the device hops to the next frequency
in sequence. This continues until SYNC is attained, or until 15
hops have passed without receiving SYNC, after which the
acquisition sequence is restarted.
5. If coarse acquisition is not obtained within 6.3 seconds, the
device selects another frequency and repeats the process beginning
with step 2.
Camping provides a worst case acquisition performance that is
perceptibly slow to the human user of a portable device. The
preferred approach has the receiver scan all potential frequencies
in ascending order, at 125 .mu.sec increments. When the highest
frequency is reached, the search begins again at the lowest
frequency. The 125 .mu.s sampling rate is much faster than the 250
.mu.sec channel switching time specification of the RF modem. This
is possible because the overall switching time specification
applies to worst case frequency switching intervals, i.e., from the
highest to the lowest operating frequency. By switching a single
channel at a time, switching may be maintained over frequency
intervals very near a synthesizer phase detectors' phase lock
range, allowing nearly instantaneous frequency switching. The
change from highest to lowest frequency at the end of the scan
requires the standard 250 .mu.sec.
The 125 .mu.sec monitoring interval allows 85 .mu.s to ascertain if
receive clock has been detected prior to switching to the next
frequency. The monitoring interval should be selected to be
non-periodic with respect to the access interval. For example, the
125 .mu.sec interval allows the entire hopping sequence to be
scanned 2(n+1) times in a 20 ms access interval.
If clock is recovered at any frequency, the receiver remains on
frequency for a Reservation Opportunity and initiates channel
access through the procedure described above. The scanning approach
is less deterministic in terms of acquisition probability than
camping, but the search time required for 99.5% acquisition
probability is about 80 Access Intervals, or three times faster
than that for camping.
A hybrid approach that scans only three or four consecutive
frequencies incorporates the deterministic aspects of camping with
some of the improved performance of the scanning algorithm. For
scanning over a small number of frequencies an up/down scan is
preferred, i.e., 1,2,3,2,1,2,3 since all frequency changes can be
accomplished at the faster switching rate. The end frequencies are
visited less often than those in the center. The number of
frequencies used, e.g., 3 or 4, is selected so that all can be
scanned during the preamble duration of a minimum length
transmission.
All devices are required to have unique 48 bit global addresses.
Local 16 bit addresses will be assigned for reduced overhead in
communications. Local addresses will not be assigned to devices
whose global addresses are not on an authentication list maintained
in each base station and routinely updated over the
infrastructure.
Once a device has attained synchronization, it must register with
the control point to be connected with the NET. It initiates this
by sending a Request for Poll indicating a registration request,
and including its global address. The control point will register
the device, and provide a short. Network Address as an outbound
message. The Control point will generate the short address if it is
a single NET, or exchange the global address for a short Network
Address with a Network Address Server if the NET is part of a
larger infrastructured network of a premises LAN.
Once a device is synchronized to a NET, it must periodically update
its local clock to the NET clock communicated in the SYNC message.
The SYNC message contains a character designated as the SYNC
character that transfers the NET clock synchronization. This may be
the beginning or ending FLAG in the SYNC message, or a specific
character within the message.
The maximum expected frequency error between NET and device local
clocks is 100 parts per million. To maintain a 50 .mu.s maximum
clock error, the local device clock must be re-synchronized at 500
ms intervals. At 20 ms per access interval, a non-sleeping device
has up to 26 SYNC opportunities within that period in which to
re-synchronize and maintain required accuracy.
As mentioned above, it is desirable that battery powered devices
have the capability to sleep, or power off, for extended periods of
time to conserve power. The term sleeping terminal in this instance
may refer to a device that powers down its radio communication
hardware to save power while maintaining other functions in an
operational state, or a device that power manages those functions
as well. In the power managed state, the device must maintain its
hop clock so that full acquisition is not required every time power
management is invoked.
Devices that must sleep to manage their power consumption use
Priority SYNC Messages to maintain synchronization. Priority SYNC
Messages occur every three Access Intervals. In times of low NET
activity, non-priority SYNC messages are omitted. By coordinating
power management with Priority SYNC Messages, power managed devices
can be guaranteed to wake up for Access Intervals where SYNCs will
be present, even if the NET activity is low during the sleep
period.
A sleeping device with no transmission requirements may sleep for
eight 20 ms access intervals, and wake only for the SYNC and
Reservation Poll at the beginning of the ninth Access Interval to
monitor pending messages before returning to the sleep state, for a
duty cycle of less than 5%. This provides three opportunities to
synchronize to the NET clock within a 540 ms window. A flow chart
depicting the a device sleeping for several access intervals is
shown in FIG. 17.
Devices may also sleep for longer periods of time, at the risk of
losing fine synchronization. They may compensate by advancing their
local clocks to account for the maximum timing uncertainty. For
example, a terminal could sleep for 5 seconds without
re-synchronizing by waking up 500 microseconds before it expects an
Access Interval to begin, and successfully receive SYNC messages.
This technique is valid for extended periods of time, up to the
point where the maximum timing error approaches 50% of an Access
Interval. A flow chart depicting the a device sleeping for several
seconds is shown in FIG. 18.
A power managed device that requires communication during a sleep
period may immediately wake and attempt access to the NET at the
next available Reservation Opportunity.
A device requiring communications may be able to register with one
of several NETs operating in its vicinity, with transmissions
occurring on many frequencies simultaneously. A good strategy is to
synchronize to a NET that provides an acceptable communication
link, then monitor HELLO messages to determine other candidate NETs
before attaching to a particular NET by registering with the
control point device.
As described above, a spontaneous wireless local area network or
spontaneous LAN is one that is established for a limited time for a
specific purpose, and which does not use the premises LAN to
facilitate communications between devices or provide access to
outside resources. Use of a spontaneous LAN allows portable devices
to share information, files, data, etc., in environments where
communication via the premises LAN is not economically justifiable
or physically possible. A spontaneous LAN capability also allows
portable/mobile devices to have an equally portable network.
Peripheral and vehicular LANs are examples of such spontaneous
LANs.
Requirements for a spontaneous LAN differ from an infrastructured
premises LAN in several significant areas. The number of devices in
a spontaneous LAN is likely to be smaller than the number that a
single NET in a premises LAN must be capable of supporting. In
addition, coverage areas for spontaneous LANs are typically smaller
than coverage areas for a base station participating in the
premises LAN. In a spontaneous LAN, communication often takes place
over relatively short distances, where devices are within line of
sight of each other.
In a premises LAN, the majority of communications are likely to
involve accessing communication network resources. For example,
portable devices with limited processing capabilities, memory, and
power supplies are able to access large databases or powerful
computing engines connected to the AC power grid. Base stations
within the premises LAN are well suited to the role of Control
Points for managing synchronization and media access within each
NET.
In a spontaneous LAN, however, communications are limited to
exchanges with spontaneous NET constituents. Additionally, NET
constituents may potentially leave at any time, making it difficult
to assign control point responsibilities to a single device. A
shared mechanism for synchronization and media access is preferable
in most cases.
In a spontaneous LAN, battery power limitations may preclude
assignment of a single device as a control point. The routine
transmission of SYNC and access control messages places a
significant power drain on a portable, battery powered device.
Also, the control point architecture dictates that transmissions
intended for devices other than the control point be stored and
forwarded to the destination device, further increasing battery
drain, and reducing system throughput.
Moreover, the use of scheduled transmission in a premises LAN is
likely to differ from use in a spontaneous LAN. For example, unlike
the premises LAN, in the spontaneous LAN, applications such as
massaging and two way voice communications may only occasionally be
used, whereas video transmission and telemetry exchange may be
prevalent.
To promote compatibility and integration with the premises LAN,
operational differences required by multiple participating devices
should be minimized. For example, selecting relatively close
frequency bands for each LAN aids in the design of a multiple LAN
transceiver, reducing circuitry, cost, power, weight and size while
increasing reliability. Similarly, selecting communication
protocols so that the spontaneous LAN protocol constitutes a subset
or superset of premises LAN may enable a given device to more
effectively communication in both LANs, while minimizing both the
overall protocol complexity and potentially limited memory and
processing power.
Use of frequency hopping is desirable in premises LAN because of
its ability to mitigate the effects of interference and frequency
selective fading. In the case of the latter, frequency hopping
allows systems to be installed with less fade margin than single
frequency systems with otherwise identical radio modem
characteristics, providing improved coverage.
The potentially smaller coverage area requirement of spontaneous
LANs, however, allows single frequency operation to be considered
for some applications, e.g., such as a peripheral LAN. Regulatory
structures are in place in some countries to allow single frequency
operation in the same bands as frequency hopping systems, providing
that single frequency devices operate at reduced power levels. The
lower transmit power of single frequency operation and elimination
of periodic channel switching are desirable methods of reducing
battery drain. The choice of single frequency or frequency hopped
operation is dictated by the coverage requirements of the network,
and may be left as an option to device users.
As noted earlier, the basic Access Interval structure is suited to
single frequency operation as well as to frequency hopping. SYNC
messages in a single frequency system substitute a single frequency
indication in the hopping sequence identifier field.
A spontaneous LAN comes into existence when two or more devices
establish communications, and ceases when its population falls to
less than two. Before a spontaneous LAN can be established, at
least two devices must agree upon a set of operating parameters for
the network. Such agreement may be pre-programmed else exchanged
and acknowledged prior to establishing the spontaneous LAN. Once
the spontaneous LAN is established, other devices coming into the
network must be able to obtain the operating parameters and acquire
access.
More specifically, to establish a spontaneous LAN, a computing
device must first identify at least one other network device with
which spontaneous LAN communication is desired. To identify another
network device, the computing device may play an active or passive
role. In an active role, the computing device periodically
broadcasts a request to form spontaneous LAN with either a specific
network device or, more likely, with a specific type of network
device. If a network device fitting the description of the request
happens to be in range or happens into range and is available, it
responds to the periodic requests to bind with the computing
device, establishing the spontaneous LAN. Alternately, the network
device may take a passive role in establishing the spontaneous LAN.
In a passive role, the computing device merely listens for a
request to form a spontaneous LAN transmitted by the appropriate
network device. Once such a network device comes into range, the
computing device responds to bind with the network device,
establishing the spontaneous LAN.
The choice of whether a device should take a passive or active role
is a matter of design choice. For example, in one embodiment where
peripheral devices have access to AC power, the roaming computer
terminals take a passive role, while the peripheral devices take a
more active role. Similarly, in another embodiment where a vehicle
terminal has access to a relatively larger battery source, an
active role is taken when attempting to form a spontaneous LAN,
i.e., a vehicular LAN, with a hand-held computing device.
Binding, a process carried out pursuant to a binding protocol
stored in each network device, may be a very simple process such as
might exist when creating a spontaneous LANs that operates on a
single frequency channel. Under such a scenario, a simple
acknowledge handshake between the computing terminal and the other
network device may be sufficient to establish a spontaneous LAN
pursuant to commonly stored, pre-programmed operating parameters.
However, more complex binding schemes may also be implemented so as
to support correspondingly more complex spontaneous LANs as proves
necessary. An example of a more complex binding scheme is described
below.
It is desirable in some large spontaneous LANs for one device to be
designated as a fully functional control point, providing identical
NET operation to a single NET in the premises LAN. Providing that
all devices share a hopping table and encryption key, the
designated device would initiate control point activities, and
other devices would synchronize to the designated unit. A device
with greater battery capacity, or one that can be temporarily
connected to AC power is best suited to the dedicated control point
function. This architecture is applicable to Client-Server
applications (where the server assumes the control point function),
or to other applications where a single device is the predominant
source or destination of communications. A portable device used as
a dedicated control point is required to have additional
programming and memory capacity to manage reservation based media
access, pending message lists, and scheduled service slot
allocations.
In embodiments where communication requirements of a spontaneous
LAN are largely peer to peer, there may be no overwhelming
candidate for a dedicated Control Point. Thus, in such cases, the
Control Point function is either distributed among some or all the
devices within the spontaneous LAN. In such scenarios, the
interleaved Access Interval approach used for wireless base
stations is employed. Initially, control point responsibilities are
determined during the binding process. Users may designate or
redesignate a Control Point device when several candidates are
available.
For spontaneous LANs, access intervals may be simplified to reduce
power consumption, program storage and processing power
requirements for portable devices used as control points. Control
Point devices transmit SYNC, pending message lists, and Time
Division Multiple Access slot reservations normally, but only use
the single slot reservation Poll (Idle Sense Multiple Access). The
reservation poll contains a field indicating reduced control point
functionality. This places other devices in a point-to-point
communication mode, using the Implicit Idle Sense Algorithm. The
probability factor p communicated in the reservation poll is used
for the Implicit Idle Sense algorithm. Control point devices may
use the deferred SYNC mechanism for light system loading,
transmitting Priority SYNC every third Access Interval to further
decrease their transmission requirements. Control point devices
must monitor the reservation slot for messages addressed to them,
but may sleep afterwards.
Request for Polls initiated under Implicit Idle Sense use
point-to-point addressing, indicating the address of the
destination device directly, rather than the control point device.
This eliminates the need for the Control Point device to store and
forward transmissions within the spontaneous LAN. The device
detecting its address in a Request for Poll begins a session, after
employing the Implicit Idle Sense algorithm, by Polling the source
address identified in the Request for Poll. The terminating ACK and
CLEAR messages contain an Explicit Idle Sense probability factor
equal to that in the original reservation poll.
To allow for power managed devices, the Control Point device
maintains a pending message list. Devices that have been unable to
establish communication with a sleeping device initiate a session
with the Control Point device to register the pending message. Upon
becoming active, the sleeping device will initiate a Poll to the
device originating the pending message. The Control Point device
will eliminate the pending message indication by aging, or by
receipt of communication from the destination device clearing the
pending message. Control point devices are not required to store
pending messages, only addresses.
As mentioned above, HELLO messages are broadcast to indicate
changes in NET parameters. HELLO messages may be omitted to
simplify the Control Point function in spontaneous LANs.
Devices are assigned local addresses upon registration with the
Control Point device. Devices may communicate an alias that
identifies the device user to other users to the Control Point
device where it is stored in an address table. The address table
may be obtained by other network constituents by querying the
Control Point device. A peripheral LAN is a type of spontaneous LAN
which serves as a short range interconnect between a portable or
mobile computing device (MCD) and peripheral devices.
Designers of portable products are constantly challenged with
reducing size, weight, and power consumption of these devices,
while at the same time increasing their functionality and improving
user ergonomics. Functions that may be used infrequently, or which
are too large to fit within the constraints of good ergonomic
design may be provided in peripheral devices, including printers,
measurement and data acquisition units, optical scanners, etc. When
cabled or otherwise physically connected to a portable product,
these peripherals often encumber the user, preventing freedom of
movement or mobility. This becomes more problematic when use of
more than one peripheral is required.
A second consideration for portable product design is communication
docking. A communication dock is a device that holsters or houses a
portable unit, and provides for communication interconnection for
such tasks as program downloading, data uploading, or communication
with large printers, such as those used for printing full sized
invoices in vehicular applications. Communication docking of a
portable unit may also involve power supply sharing and/or
charging.
The requirement for communication docking capability forces newer
portable product designs to be mechanically compatible with older
docking schemes, or may require that new docks, or adapters, be
developed for each new generation of portable device. Product
specific docking approaches eliminate compatibility between devices
manufactured by different suppliers. This has hindered development
of uniform standards for Electronic Data Interchange between
portable devices and fixed computing systems.
Physical connection between a portable device with a peripheral or
communication dock also hinders user efficiency. Peripheral devices
are generally attached with cable. If a peripheral is small enough
to be carried or worn on a belt, the mobility of the user may be
maintained. If a user must carry a hand-held portable device that
is connected to a belt mounted peripheral the assembly cannot be
set down while a task that requires movement to a location several
feet away is undertaken unless the portable device and peripheral
are disconnected. Likewise, connection to peripherals too large to
be portable requires the user to frequently connect and disconnect
the device and the peripheral.
Use of wireless peripheral LAN interconnection greatly simplifies
the task of portable devices communicating with peripherals. In
doing so, wireless connectivity allows improved ergonomics in
portable product design, flexibility in interconnection to one or
more peripherals, freedom of movement over a radius of operation,
forward and backward compatibility between portable units and
peripherals, and potential communications among products
manufactured by different vendors.
Constituents within a peripheral LAN generally number six or fewer
devices. One roaming computing device and one or two peripherals
comprise a typical configuration. Operating range is typically less
than fifty feet.
Because the computing devices generally control the operation of
peripheral devices, in a peripheral LAN a master/slave type
protocol is appropriate. Moreover, roaming computing devices
serving as master are well suited to the role of Control Points for
managing synchronization and media access within each peripheral
LAN. All peripheral communications are slaved to the master.
In a peripheral LAN, roaming mobile or portable computing devices
and wireless peripherals may all operate from battery power.
Operating cycles between charging dictate use of power management
techniques.
Although all participants in a peripheral LAN might also be
configured to directly participate in the premises LAN, the
trade-offs in cost, power usage and added complexity often times
weighs against such configuration. Even so, participants within a
peripheral LAN can be expected to function in a hierarchical
manner, through a multiple participating device, with the premises
LAN. Thus, the use of a much simpler, lower-power transceiver and
associated protocol may be used in the peripheral LAN.
As previously described, a roaming computing device serving as a
master device may itself be simultaneously attempting to
participate in other networks such as the premises or vehicular
LANs. Considerable benefits arise if the radio and processing
hardware that supports operation within the wireless network can
also support such operation. For example, a device that is capable
of frequency hopping is inherently suited to single frequency
operation. If it can adjust transmitter power level and data rate
to be compatible with the requirements of the peripherals LAN, it
can function in both systems. The major benefits of common
transceiver hardware across LANs include smaller product size,
improved ergonomics, and lower cost.
Specifically, in one embodiment, radio communication on the
premises LAN, as described herein, takes place using radio
transceivers capable of performing frequency-hopping. To
communicate on a peripheral LAN, such transceivers could also
utilize frequency-hopping at a lower power. However, such
transceivers are relatively expensive in comparison to a lower
power, narrow-band, single frequency transceivers. Because of the
cost differential, it proves desirable to use the single frequency
transceivers for all peripheral devices which will not participate
in the premises LAN. Therefore, the more expensive,
frequency-hopping transceivers which are fitted into roaming
computing devices are further designed to stop hopping and lock
into the frequency of the single frequency transceiver, allowing
the establishment of peripheral LANs.
Instead of frequency hopping, the peripheral LAN may also use
narrow-band, single frequency communication, further simplifying
the radio transceiver design for commonality. In another embodiment
of the peripheral LAN transceivers, operation using one of a
plurality of single frequency channels is provided. Thus, to
overcome interference on one channel, the transceiver might select
from the remaining of the plurality an alternate, single operating
frequency with lesser channel interference. To accommodate the
plurality of single frequency channels, the peripheral LAN
transceivers may either communicate an upcoming frequency change so
that corresponding peripheral LAN participants can also change
frequency, or the transceivers may be configured to use frequency
synthesis techniques to determine which of the plurality a current
transmission happens to be.
The Access Interval structure is also an appropriate choice for
peripheral LAN operations. In one embodiment, to provide for
simplicity and tighter integration, the Access Interval for the
peripheral LAN is a subset of the Access Interval used in the
premises LAN. HELLO messages, Implicit Idle Sense, Data Rate
Switching, and scheduled services are not implemented. Peripheral
devices normally sleep, activate their receivers for SYNC
transmissions from the participating master device, and resume
sleeping if no pending messages are indicated and they have no
inbound transmission requirements. Access Intervals occur at
regular intervals, allowing for power management. Access Intervals
may be skipped if the master has other priority tasks to
complete.
To initialize the peripheral LAN, a device desiring initialization,
a master device, selects a single operating frequency by scanning
the available frequencies for one with no activity. A typical
master device might be a roaming computing device desiring access
to a local peripheral. Default values for other parameters,
including Access Interval duration, are contained within each
participant's memory. Such parameters may be pre-adjusted in each
participant to yield specific performance characteristics in the
peripheral LAN.
Once a master device identifies a single frequency, slaves, which
are generally peripherals, are brought into the peripheral LAN
through a process called binding. Binding is initiated by the
master device by invoking a binding program contained therein.
Slaves, such as peripherals, are generally programmed to enter a
receptive state when idle. Thus, in one embodiment, the master
device accomplishes binding by transmitting Access Intervals of
known duration sequentially on a series of four frequencies spread
throughout the available frequency range. The specific frequencies
and Access Interval durations used are stored as parameters in all
potential participating devices. A 250 KBPS transfer rate is
appropriate in some embodiments of the peripheral LAN, reflecting a
balance between performance and complexity in peripheral
devices.
A slave, e.g., a peripheral, responds to the binding attempts by
the master device on a given frequency until the slave successfully
receives and establishes communication with the master device. If
they do not establish communication after four Access Intervals,
the slave switches to the next frequency for four Access Interval
periods. Once communication is established, the slave registers
with the master and obtains the master device's selected operating
frequency and related communication parameters. When all slave
devices have been bound, the master terminates the binding program
and normal operation at the selected single frequency may
begin.
Referring to FIG. 15, in a hierarchical network, peripheral LAN
masters use a secondary access interval 1501 that is synchronized
to the Access Interval of a parent (premises) LAN control point.
Peripheral LAN Access Intervals occur less frequently than premises
LAN Access Intervals, e.g., every other or every third Priority
SYNC Access Interval.
During the premises LAN Access Interval, the peripheral LAN master
device monitors the premises LAN control point for SYNC 1503
reservation poll 1505 and exchanges inbound and outbound message
according to the normal rules of the access protocol. The master
switches to the peripheral LAN frequency, and transmits its own
SYNC frame 1507 during the session period 1509 of its parent
control point allowing communication with its peripherals. The
peripheral LAN Access Interval is generally shorter than the
premises LAN Access Interval, so that it does not extend beyond the
premises LAN Access Interval boundary. At the end of the peripheral
LAN Access Interval 1501, the master switches to the premises LAN
frequency for the next SYNC 1503.
The secondary SYNC 1507 may only be transmitted if the peripheral
LAN master is not busy communicating through the premises LAN. If a
communication session is occurring, the master must defer SYNC,
preventing communication with its peripherals during that Access
Interval. The master must also defer SYNC if the current frequency
in the LAN is prone to interference from the peripheral LAN
frequency, i.e., they are the same frequency or adjacent
frequencies. If two consecutive SYNCs are deferred, peripherals
will activate their receivers continuously for a period of time,
allowing the master to transmit during any Access Interval. This
approach is also applicable when the master roams between frequency
hopping NETs. Since NETs are not synchronized to one another, the
devices in the peripheral LAN adjust Access Interval boundaries
each time the master roams. If peripherals do not detect SYNC
within a time-out period, they may duty cycle their reception to
conserve battery power.
Referring to FIG. 16, a Roaming Algorithm Flow Diagram illustrates
how a roaming computing device will select a suitable base station.
Roaming computing devices operating in the infrastructured network
environment formed by the base stations will routinely move between
base station coverage areas. The roaming computing devices are able
to disconnect from their current base station communication link
and reconnect a communication link to a different base station, as
necessitated by device roaming.
Base stations transmit HELLO messages to devices in their coverage
area. These HELLO messages communicate to roaming computing devices
the cost of connection through the base station, addresses of
neighboring base stations, and the cost of connection through these
neighboring base stations. This information allows roaming
computing devices to determine the lowest cost connection available
and to connect to the base station with the lowest cost.
In addition, base station HELLO message may include communication
parameters of neighboring base stations, such as frequency hopping
sequences and indices, spread spectrum spreading codes, or FM
carrier channel frequencies. This information allows roaming
computing devices to roam and change base station connections
without going through a full acquisition phase of the new base
station's parameters.
Roaming computing devices initially select and register with a base
station control point on the basis of link quality: signal strength
and cost information transmitted within HELLO messages. A device
will remain attached to a particular base station until the link
quality degrades below an acceptable level; then it will attempt to
determine if an alternative base station connection is available.
The device initiates a roaming algorithm, using neighbors
information from the most recent HELLO message to attempt
connection with another candidate base station. If connection
fails, another candidate from the neighbors list will be selected,
and the process repeated. Once connection is made with an
alternative base station, the device will monitor signal strength
and data errors to determine link quality. If link quality is
acceptable, the device will continue monitoring until a HELLO
message is received. If the cost is acceptable, it will register
with the new base station, and the base station will update the
spanning tree over the infrastructure. If link quality or cost is
unacceptable, another candidate from the neighbors list is selected
and the process repeated. This continues until an acceptable
connection is established. If one cannot be established, the device
must return to the original base station connection or employ the
initial acquisition algorithm.
FIG. 28a illustrates an embodiment of the hierarchical
communication system whereby communication is maintained in a
warehouse environment. Specifically, a worker utilizes a roaming
computing device, a computer terminal 3007, and a code reader 3009
to collect data such as identifying numbers or codes on warehoused
goods, such as the box 3010. As the numbers and codes are
collected, they are forwarded through the network to a host
computer 3011 for storage and cross-referencing. In addition, the
host computer 3011 may, for example, forward cross-referenced
information relating to the collected numbers or codes back through
the network for display on the terminal 3007 or for priming on a
printer 3013. Similarly, the collected information may be printed
from the computer terminal 3007 directly on the printer 3013. Other
exemplary communication pathways supported include message
exchanges between the computer terminal 3007 and other computer
terminals (not shown) or the host computer 3011.
The host computer 3011 provides the terminal 3007 with remote
database storage, access and processing. However, the terminal 3007
also provides for local processing within its architecture to
minimize the need to access the remote host computer 3011. For
example, the terminal 3007 may store a local database for local
processing. Similarly, the terminal 3007 may run a variety of
application programs which never, occasionally or often need access
to the remote host computer 3011.
Many of the devices found in the illustrative network are battery
powered and therefore must conservatively utilize their radio
transceivers. For example, the hand-held computer terminal 3007
receives its power from either an enclosed battery or a forklift
battery (not shown) via a communication dock within the forklift
3014. Similarly, the code reader 3009 operates on portable battery
power as may the printer 3013. The arrangement of the communication
network, communication protocols used, and data rate and power
level adjustments help to optimize battery conservation without
substantially degrading network performance.
In the illustrated embodiment shown in FIG. 28a, the hierarchical
communication system consists of a premises LAN covering a building
or group of buildings. The premises LAN in the illustrated
embodiment includes a hard-wired backbone LAN 3019 and base
stations 3015 and 3017. A host computer 3011 and any other
non-mobile network device located in the vicinity of the backbone
LAN 3019 can be directly attached to the backbone LAN 3019.
However, mobile devices and remotely located devices must maintain
connectivity to the backbone LAN 3019 through either a single base
station such as the base station 3015, or through a multi-hop
network of base stations such as is illustrated by the base
stations 3015 and 3017. The base stations 3015 and 3017 contain a
relatively higher power transmitter, and provide coverage over the
entire warehouse floor. Although a single base station may be
sufficient, if the warehouse is too large or contains interfering
physical barriers, the multi-hop plurality of base stations 3017
may be desirable. Otherwise, the backbone LAN 3019 must be extended
to connect all of the base stations 3017 directly to provide
sufficient radio coverage. Through the premises LAN, relatively
stable, longer range wireless and hard-wired communication is
maintained.
Because roaming computing devices, such as the hand-held computer
terminal 3007, cannot be directly hard-wired to the backbone LAN
3019, they are fitted with RF transceivers. To guarantee that such
a network device can directly communicate on the premises LAN with
at least one of the base stations 3015 and 3017, the fitted
transceiver is selected to yield approximately the same
transmission power as do the base stations 3015 and 3017. However,
not all roaming network devices require a direct RF link to the
base stations 3015 and 3017, and some may not require any link at
all. Instead, with such devices, communication exchange is
generally localized to a small area and, as such, only requires the
use of relatively lower power, short range transceivers. The
devices which participate in such localized, shorter range
communication form spontaneous LANs.
For example, the desire by a roaming terminal to access peripheral
devices such as the printer 3013 and modem 3023, results in the
roaming terminal establishing a peripheral LAN with the peripheral
devices. Similarly, a peripheral LAN might be established when
needed to maintain local communication between a code scanner 3009
and the terminal 3007. In an exemplary embodiment, the printer 3013
are located in a warehouse dock with the sole assignment of
printing out forms based on the code information gathered from
boxes delivered to the dock. In particular, as soon as the code
reader gathers information, it relays the information along a
peripheral LAN to the terminal 3007. Upon receipt, the terminal
3007 communicates via the premises LAN to the host computer 3011 to
gather related information regarding a given box. Upon receipt of
the related information, the terminal 3007 determines that printing
is desired with the printer 3013 located at the dock. When the
forklift 3014 enters the vicinity of the dock, the terminal 3007
establishes a peripheral LAN with the printer 3013 which begins
printing the collected code information.
To carry out the previous communication exchange, the printer 3013
and code reader 3009 are fitted with a lower power peripheral LAN
transceivers for short range communication. The computer terminal
3007 transceiver is not only capable of peripheral LAN
communication, but also with the capability of maintaining premises
LAN communication. In an alternate exchange however, the code
reader 3009 might be configured to participate on both LANs, so
that the code reader 3009 participates in the premises LAN to
request associated code information from the host computer 3011. In
such a configuration, either the code reader 3009 or terminal 3007
could act as the control point of the peripheral LAN. Alternately,
both could share the task.
With capability to participate in the peripheral LAN only, the code
reader 3009, or any other peripheral LAN participant, might still
gain access to the premises LAN indirectly through the terminal
3007 acting as a relaying device. For example, to reach the host
computer 3011, the code reader 3009 first transmits to the computer
terminal 3007 via the peripheral LAN. Upon receipt, the computer
terminal 3007 relays the transmission to one of the base stations
3015 and 3017 for forwarding to the host 3011. Communication from
the host 3011 to the code reader 3009 is accomplished via the same
pathway.
It is also possible for any two devices with no access to the
premises LAN to communicate to each other. For example, the modem
3023 could receive data and directly transmit it for printing to
the printer 3013 via a peripheral LAN established between the two.
Similarly, the code reader 3009 might choose to directly
communicate code signals through a peripheral LAN to other network
devices via the modem 3023.
In an alternate configuration, a peripheral LAN base station 3021
is provided which may be directly connected to the backbone LAN
3019 (as shown) or indirectly connected via the base stations 3015
and 3017. The peripheral LAN base station 3021 is positioned in the
vicinity of other peripheral LAN devices and thereafter becomes a
control point participant. Thus, peripheral LAN communication
flowing to or from the premises LAN avoids high power radio
transmissions altogether. However, it can be appreciated that a
stationary peripheral LAN base station may not always be an option
when all of the peripheral LAN participants are mobile. In such
cases, a high power transmission to reach the premises LAN may be
required.
FIG. 28b illustrates other features in the use of spontaneous LANs
in association with a vehicle which illustrate the capability of
automatically establishing a premises and a peripheral LAN when
moving in and out of range to perform services and report on
services rendered. In particular, like the forklift 3014 of FIG.
28a, a delivery truck 3033 provides a focal point for a spontaneous
LAN utilization. Within the truck 3033, a storage terminal 3031 is
docked so as to draw power from the truck 3033's battery supply.
Similarly, a computer terminal 3007 may either be docked or ported.
Because of greater battery access, the storage terminal 3031 need
only be configured for multiple participation in the premises,
peripheral and vehicular LANs and in a radio WAN, such as RAM
Mobile Data, CDPD, MTEL, ARDIS, etc. The storage terminal 3031,
although also capable of premises and peripheral LAN participation,
need only be configured for vehicular LAN participation.
Prior to making a delivery, the truck enters a docking area for
loading. As goods are loaded into the truck, the information
regarding the goods is down-loaded into the storage terminal 3031
via the terminal 3007 or code reader 3009 (FIG. 28a) via the
premises or peripheral LAN communications. This loading might also
be accomplished automatically as the forklift 3014 comes into range
of the delivery truck 3033, establishes or joins the peripheral
LAN, and transmits the previously collected data as described above
in relation to FIG. 28a. Alternately, loading might also be
accomplished via the premises LAN.
As information regarding a good is received and stored, the storage
terminal 3031 might also request further information regarding any
or all of the goods via the peripheral LAN's link to the host
computer 3011 through the premises LAN. More likely however, the
storage terminal 3031 if appropriately configured would participate
on the premises LAN to communicate directly with the host computer
3011 to retrieve such information.
The peripheral LAN base station 3021 if located on the dock could
provide a direct low power peripheral LAN connection to the
backbone LAN 3019 and to the host computer 3011. Once fully loaded
and prior to leaving the dock, the storage device 3031 may generate
a printout of the information relating to the loaded goods via a
peripheral LAN established with the printer 3013 on the dock. In
addition, the information may be transmitted via the peripheral LAN
modem 3023 to a given destination site.
As illustrated in FIG. 28c, once the storage terminal 3031 and
hand-held terminal 3007 moves out of range of the premises and
peripheral LANs, i.e., the truck 3033 drives away from the dock,
the vehicular LAN can only gain access to the premises LAN via the
more costly radio WAN communication. Thus, although the storage
terminal 3031 might only be configured with relaying control point
functionality, to minimize radio WAN communication, the storage
terminal 3031 can be configured to store relatively large amounts
of information and to provide processing power. Thus, the terminal
3007 can access such information and processing power without
having to access devices on the premises LAN via the radio WAN.
Upon reaching the destination, the storage terminal 3031 may
participate in any in range peripheral and premises LAN at the
delivery site dock. Specifically, as specific goods are unloaded,
they are scanned for delivery verification, preventing delivery of
unwanted goods. The driver is also informed if goods that should
have been delivered are still in the truck. As this process takes
place, a report can also be generated via a peripheral or premises
LAN printer at the destination dock for receipt signature.
Similarly, the peripheral LAN modem on the destination dock can
relay the delivery information back to the host computer 3011 for
billing information or gather additional information needed,
avoiding use of the radio WAN.
If the truck 3033 is used for service purposes, the truck 3033
leaves the dock in the morning with the addresses and directions of
the service destinations, technical manuals, and service notes
which have been selectively downloaded from the host computer 3011
via either the premises or peripheral LAN to the storage terminal
3031 which may be configured with a hard drive and substantial
processing power. Upon pulling out of range, the storage terminal
3031 and the computer terminal 3007 automatically form an
independent, detached vehicular LAN. Alternately, the terminals
3007 and 3031 may have previously formed the vehicular LAN before
leaving dock. In one embodiment, the vehicular LAN operates using
frequency hopping protocol much the same as that of the premises
LAN, with the storage terminal 3031 acting much like the premises
LAN base stations. Thus, the radio transceiver circuitry for the
premises LAN participation may also be used for the vehicular LAN
and, as detailed above, a peripheral LAN. Similarly, if the radio
WAN chosen has similar characteristics, it may be incorporated into
a single radio transceiver.
At each service address, the driver collects information using the
terminal 3007 either as the data is collected, if within vehicular
LAN transmission range of the storage terminal 3031, or as soon as
the terminal 3007 comes within range. Any stored information within
storage terminal 3031 may be requested via the vehicular LAN by the
hand-held terminal 3007. Information not stored within the
vehicular LAN may be communicated via a radio WAN as described
above.
Referring again to FIG. 28b, upon returning to the dock, the
storage terminal 3031, also referred to herein as a vehicle
terminal, joins in or establishes a peripheral LAN with the
peripheral LAN devices on the dock, if necessary. Communication is
also established via the premises LAN. Thereafter, the storage
terminal 3031 automatically transfers the service information to
the host computer 3011 which uses the information for billing and
in formulating service destinations for automatic downloading the
next day.
FIG. 29 is a diagrammatic illustration of another embodiment using
a peripheral LAN to supporting roaming data collection by an
operator according to the present invention. As an operator 3061
roams the warehouse floor he carries with him a peripheral LAN
comprising the terminal 3007, code reader 3009 and a portable
printer 3058. The operator collects information regarding goods,
such as the box 3010, with the code reader 3009 and the terminal
3007. If the power resources are equal, the terminal 3007 may be
configured and designated to also participate in the premises
LAN.
Corresponding information to the code data must be retrieved from
the host computer 3011. The collected code information and
retrieved corresponding information can be displayed on the
terminal 3007. After viewing for verification, the information can
be printed on the printer 3058. Because of this data flow
requirement, the computer terminal 3007 is selected as the
peripheral LAN device which must also carry the responsibility of
communicating with the premises LAN.
If during collection, the operator decides to power down the
computer terminal 3007 because it is not needed, the peripheral LAN
becomes detached from the premises LAN. Although it might be
possible for the detached peripheral LAN to function, all
communication with the host computer 3011 through the premises LAN
is placed in a queue awaiting reattachment. As soon as the detached
peripheral LAN comes within range of an attached peripheral LAN
device, i.e., a device attached to the premises LAN, the queued
communications are relayed to the host. It should be clear from
this description that the peripheral LAN may roam in relation to a
device attached to the premises LAN ("premises LAN device").
Similarly, the premises LAN device may roam in relation to the
peripheral LAN. The roaming constitutes a relative positioning.
Moreover, whenever a peripheral LAN and a master device move out of
range of each other, the peripheral LAN may either poll for or scan
for another master device for attachment. The master device may
constitute a premises LAN device, yet need not be.
To avoid detachment when the terminal 3007 is powered down, the
code reader 3009 may be designated as a backup to the terminal 3007
for performing the higher power communication with the premises
LAN. As described in more detail below in reference to FIG. 33c
regarding the idle sense protocol, whenever the code reader 3009
determines that the terminal 3007 has stopped providing access to
the premises LAN, the code reader 3009 will take over the role if
it is next in line to perform the backup service. Thereafter, when
the computer terminal 3007 is powered up, it monitors the
peripheral LAN channel, requests and regains from the code reader
3009 the role of providing an interface with the premises LAN.
This, however, does not restrict the code reader 3009 from
accessing the premises LAN although the reader 3009 may choose to
use the computer terminal 3007 for power conservation reasons.
In addition, if the computer terminal 3007 reaches a predetermined
low battery threshold level, the terminal 3007 will attempt to pass
the burden of providing premises LAN access to other peripheral LAN
backup devices. If no backup device exists in the current
peripheral LAN, the computer terminal 3007 may refuse all high
power transmissions to the premises LAN. Alternatively, the
computer terminal 3007 may either refuse predetermined select types
of requests, or prompt the operator before performing any
transmission to the premises LAN. However, the computer terminal
3007 may still listen to the communications from the premises LAN
and inform peripheral LAN members of waiting messages.
FIG. 30 is a block diagram illustrating the functionality of RF
transceivers built to communicate in the hierarchical network.
Although preferably plugging into PCMCIA slots of the computer
terminals and peripherals, the transceiver 3110 may also be
built-in or externally attached via available serial, parallel or
ethernet connectors for example. Although the transceivers used by
potential peripheral LAN master devices may vary from those used by
peripheral LAN slave devices (as detailed below), they all contain
the illustrated functional blocks.
In particular, the transceiver 3110 contains a radio unit 3112
which attaches to an attached antenna 3113. The radio unit 3112
used in peripheral LAN slave devices need only provide reliable low
power transmissions, and are designed to conserve cost, weight and
size. Potential peripheral LAN master devices not only require the
ability to communicate with peripheral LAN slave devices, but also
require higher power radios to also communicate with the premises
LAN. Thus, potential peripheral LAN master devices and other
non-peripheral LAN slave devices might contain two radio units 3112
or two transceivers 3110--one serving the premises LAN and the
other serving the peripheral LAN--else only contain a single radio
unit to service both networks.
In embodiments where cost and additional weight is not an issue, a
dual radio unit configuration for potential peripheral LAN master
devices may provide several advantages. For example, simultaneous
transceiver operation is possible by choosing a different operating
band for each radio. In such embodiments, a 2.4 GHz radio is
included for premises LAN communication while a 27 MHz radio
supports the peripheral LAN. Peripheral LAN slave devices receive
only the 27 MHz radio, while the non-potential peripheral LAN
participants from the premises LAN are fitted with only the 2.4 GHz
radios. Potential peripheral LAN master devices receive both
radios. The low power 27 MHz peripheral LAN radio is capable of
reliably transferring information at a range of approximately 40 to
100 feet asynchronously at 19.2 KBPS. An additional benefit of
using the 27 MHz frequency is that it is an unlicensed frequency
band. The 2.4 GHz radio provides sufficient power (up to 1 Watt) to
communicate with other premises LAN devices. Another benefit of
choosing 2.4 GHz or 27 MHz bands is that neither require FCC
licensing. Many different frequency choices could also be made such
as the 900 MHz band, etc.
In embodiments where cost and additional weight are at issue, a
single radio unit configuration is used for potential peripheral
LAN master devices. Specifically, in such embodiments, a dual mode
2.4 GHz radio supports both the peripheral LAN and premises LANs.
In a peripheral LAN mode, the 2.4 GHz radio operates at a single
frequency, low power level (sub-milliwatt) to support peripheral
LAN communication at relatively close distances 20-30 feet). In a
high power (up to 1 Watt) or main mode, the 2.4 GHz radio provides
for frequency-hopping communication over relatively long distance
communication connectivity with the premises LAN. Although all
network devices might be fitted with such a dual mode radio, only
peripheral LAN master devices use both modes. Peripheral LAN slave
devices would only use the low power mode while all other premises
LAN devices would use only the high power mode. Because of this, to
save cost, peripheral LAN slave devices are fitted with a single
mode radio operating in the peripheral LAN mode. Non-peripheral LAN
participants are also fitted with a single mode (main mode) radio
unit for cost savings.
Connected between the radio unit 3112 and an interface 3110, a
microprocessor 3120 controls the information flow between through
the transceiver 3110. Specifically, the interface 3115 connects the
transceiver 3110 to a selected computer terminal, a peripheral
device or other network device. Many different interfaces 3115 are
used and the choice will depend upon the connection port of the
device to which the transceiver 3110 will be attached. Virtually
any type of interface 3110 could be adapted for use with the
transceiver 3110 of the present invention. Common industry
interface standards include RS-232, RS-422, RS-485, 10BASE2
Ethernet, 10BASE5 Ethernet, 10BASE-T Ethernet, fiber optics, IBM
4/16 Token Ring, V.11, V.24, V.35, Apple Localtalk and telephone
interfaces. In addition, via the interface 3115, the microprocessor
3120 maintains a radio independent, interface protocol with the
attached network device, isolating the attached device from the
variations in radios being used.
The microprocessor 3120 also controls the radio unit 3112 to
accommodate communication with the either the premises LAN, the
peripheral LAN, or both (for dual mode radios). Moreover, the same
radio might also be used for vehicular LAN and radio WAN
communication as described above. More specifically, in a main mode
transceiver, the microprocessor 3120 utilizes a premises LAN
protocol to communicate with the premises LAN. Similarly, in a
peripheral LAN mode transceiver, the microprocessor 3120 operates
pursuant to a peripheral LAN protocol to communicate in the
peripheral LAN. In the dual mode transceiver, the microprocessor
3120 manages the use of and potential conflicts between both the
premises and peripheral LAN protocols. Detail regarding the
premises and peripheral LAN protocols can be found in reference to
FIGS. 33-36 below.
In addition, as directed by the corresponding communication
protocol, the microprocessor 3120 controls the power consumption of
the radio 3112, itself and the interface 3115 for power
conservation. This is accomplished in two ways. First, the
peripheral LAN and premises protocols are designed to provide for a
low power mode or sleep mode during periods when no communication
involving the subject transmitter is desired as described below in
relation to FIGS. 33-34. Second, both protocols are designed to
adapt in both data rate and transmission power based on power
supply (i.e., battery) parameters and range information as
described in reference to FIGS. 35-36.
In order to insure that the proper device is receiving the
information transmitted, each device is assigned, a unique address.
Specifically, the transceiver 3110 can either have a unique address
of its own or can use the unique address of the device to which it
is attached. The unique address of the transceiver can either be
one selected by the operator or system designer or one which is
permanently assigned at the factory such as an IEEE address. The
address 3121 of the particular transceiver 3110 is stored with the
microprocessor 3120.
In the illustrated embodiments of FIGS. 28-29, the peripheral LAN
master device is shown as being either a peripheral LAN base
station or a mobile or portable computer terminal. From a data flow
viewpoint, in considering the fastest access through the network,
such choices for the peripheral LAN master devices appear optimal.
However, any peripheral LAN device might be assigned the role of
the master, even those that do not seem to provide an optimal data
flow pathway but may provide for optimal battery usage. For
example, in the personal peripheral LAN of FIG. 29, because of the
support from the belt 3059, the printer might contain the greatest
battery capacity of the personal peripheral LAN devices. As such,
the printer might be designated the peripheral LAN master device
and be fitted with either a dual mode radio or two radios as master
devices require. The printer, or other peripheral LAN slave
devices, might also be fitted with such required radios to serve
only as a peripheral LAN master backup. If the battery power on the
actual peripheral LAN master, i.e., the hand-held terminal 3007
(FIG. 29, drops below a preset threshold, the backup master takes
over.
FIG. 31 is a drawing which illustrates an embodiment of the
personal peripheral LAN shown in FIG. 29 which designates a printer
as the peripheral LAN master device. Specifically, in a personal
peripheral LAN 3165, a computer terminal 3170 is strapped to the
forearm of the operator. A code reader 3171 straps to the back of
the hand of the user and is triggered by pressing a button 3173
with the thumb. Because of their relatively low battery energy, the
computer terminal 3170 and code reader 3171 are designated
peripheral LAN slave devices and each contain a peripheral LAN
transceiver having a broadcast range of two meters or less. Because
of its greater battery energy, the printer 3172 contains a dual
mode radio and is designated the peripheral LAN master device.
FIG. 32 is a block diagram illustrating a channel access algorithm
used by peripheral LAN slave devices. At a block 3181, when a slave
device has a message to send, it waits for an idle sense message to
be received from the peripheral LAN master device at a block 3183.
When an idle sense message is received, the slave device executes a
back-off protocol at a block 3187 in an attempt to avoid collisions
with other slave devices waiting to transmit. Basically, instead of
permitting every slave device to repeatedly transmit immediately
after an idle sense message is received, each waiting slave is
required to first wait for a pseudo-random time period before
attempting a transmission. The pseudo-random back-off time period
is generated and the waiting takes place at a block 3187. At a
block 3189, the channel is sensed to determine whether it is clear
for transmission. If not, a branch is made back to the block 3183
to attempt a transmission upon receipt of the next idle sense
message. If the channel is still clear, at a block 3191, a
relatively small "request to send" type packet is transmitted
indicating the desire to send a message. If no responsive "clear to
send" type message is received from the master device, the slave
device assumes that a collision occurred at a block 3193 and
branches back to the block 3183 to try again. If the "clear to
send" message is received, the slave device transmits the message
at a block 3195.
Several alternate channel access strategies have been developed for
carrier sense multiple access (CSMA) systems and include
1-persistent, non-persistent and p-persistent. Such strategies or
variations thereof could easily be adapted to work with the present
invention.
FIG. 33a is a timing diagram of the protocol used according to one
embodiment illustrating a typical communication exchange between a
peripheral LAN master device having virtually unlimited power
resources and a peripheral LAN slave device. Time line 3201
represents communication activity by the peripheral LAN master
device while time line 3203 represents the corresponding activity
by the peripheral LAN slave device. The master periodically
transmits an idle sense message 3205 indicating that it is
available for communication or that it has data for transmission to
a slave device. Because the master has virtually unlimited power
resources, it "stays awake" for the entire time period 3207 between
the idle sense messages 3205. In other words, the master does not
enter a power conserving mode during the time periods 3207.
The slave device uses a binding protocol (discussed below with
regard to FIG. 33c) to synchronize to the master device so that the
slave may enter a power conserving mode and still monitor the idle
sense messages of the master to determine if the master requires
servicing. For example, referring to FIG. 33a, the slave device
monitors an idle sense message of the master during a time period
3209, determines that no servicing is required, and enters a power
conserving mode during the time period 3211. The slave then
activates during a time period 3213 to monitor the next idle sense
message of the master. Again, the slave determines that no
servicing is required and enters a power conserving mode during a
time period 3215. When the slave activates again during a time
period 3217 to monitor the next idle sense message, it determines
from a "request to send" type message from the master that the
master has data for transmission to the slave. The slave responds
by sending a "clear to send" type message during the time period
3217 and stays activated in order to receive transmission of the
data. The master is thus able to transmit the data to the slave
during a time period 3219. Once the data is received by the slave
at the end of the time period 3221, the slave again enters a power
conserving mode during a time period 3223 and activates again
during the time period 3225 to monitor the next idle sense
message.
Alternatively, the slave may have data for transfer to the master.
If so, the slave indicates as such to the master by transmitting a
message during the time period 3217 and then executes a backoff
algorithm to determine how long it must wait before transmitting
the data. The slave determines from the backoff algorithm that it
must wait the time period 3227 before transmitting the data during
the time period 3221. The slave devices use the backoff algorithm
in an attempt to avoid the collision of data with that from other
slave devices which are also trying to communicate with the master.
The backoff algorithm is discussed more fully above in reference to
FIG. 32.
The idle sense messages of the master may also aid in scheduling
communication between two slave devices. For example, if a first
slave device has data for transfer to a second slave device, the
first slave sends a message to the master during the time period
3209 requesting communication with the second slave. The master
then broadcasts the request during the next idle sense message.
Because the second slave is monitoring the idle sense message, the
second slave receives the request and stays activated at the end of
the idle sense message in order to receive the communication.
Likewise, because the first slave is also monitoring the idle sense
message, it too receives the request and stays activated during the
time period 3215 to send the communication.
FIG. 33b is a timing diagram of the protocol used according to one
embodiment illustrating a typical communication exchange between a
peripheral LAN master having limited power resources and a
peripheral LAN slave device. This exchange is similar to that
illustrated in FIG. 33a except that, because it has limited power
resources, the master enters a power conserving mode. Before
transmitting an idle sense message, the master listens to determine
if the channel is idle. If the channel is idle, the master
transmits an idle sense message 3205 and then waits a time period
3231 to determine if any devices desire communication. If no
communication is desired, the master enters a power conserving mode
during a time period 3233 before activating again to listen to the
channel. If the channel is not idle, the master does not send the
idle sense message and enters a power saving mode for a time period
3235 before activating again to listen to the channel.
Communication between the master and slave devices is the same as
that discussed above in reference to FIG. 33a except that, after
sending or receiving data during the time period 3219, the master
device enters a power conserving mode during the time period
3237.
FIG. 33c is also a timing diagram of one embodiment of the protocol
which illustrates a scenario wherein the peripheral LAN master
device fails to service peripheral LAN slave devices. The master
device periodically sends an idle sense message 3205, waits a time
period 3231, and enters a power conserving mode during a time
period 3233 as discussed above in reference to FIG. 33b. Similarly,
the slave device monitors the idle sense messages during time
periods 3209 and 3213 and enters a power conserving mode during
time periods 3211 and 3215. For some reason, however, the master
stops transmitting idle sense messages. Such a situation may occur,
for example, if the master device is portable and is carried
outside the range of the slave's radio. During a time period 3241,
the slave unsuccessfully attempts to monitor an idle sense message.
The slave then goes to sleep for a time period 3243 and activates
to attempt to monitor a next idle sense message during a time
period 3245, but is again unsuccessful.
The slave device thereafter initiates a binding protocol to attempt
to regain synchronization with the master. While two time periods
3241 and 3245 are shown, the slave may initiate such a protocol
after any number of unsuccessful attempts to locate an idle sense
message. With this protocol, the slave stays active for a time
period 3247, which is equal to the time period from one idle sense
message to the next, in an attempt to locate a next idle sense
message. If the slave is again unsuccessful, it may stay active
until it locates an idle sense message from the master, or, if
power consumption is a concern, the slave may enter a power
conserving mode at the end of the time period 3247 and activate at
a later time to monitor for an idle sense message.
In the event the master device remains outside the range of the
slave devices in the peripheral LAN for a period long enough such
that communication is hindered, one of the slave devices may take
over the functionality of the master device. Such a situation is
useful when the slave devices need to communicate with each other
in the absence of the master. Preferably, such a backup device has
the ability to communicate with devices on the premises LAN. If the
original master returns, it listens to the channel to determine
idle sense messages from the backup, indicates to the backup that
it has returned and then begins idle sense transmissions when it
reestablishes dominance over the peripheral LAN.
FIG. 34 is a timing diagram illustrating one embodiment of the
peripheral LAN master device's servicing of both the high powered
premises LAN and the low powered peripheral LAN subnetwork, with a
single or plural radio transceivers. Block 3251 represents typical
communication activity of the master device. Line 3253 illustrates
the master's communication with a base station on the premises LAN
while line 3255 illustrates the master's communication with a slave
device on the peripheral LAN. Lines 3257 and 3259 illustrate
corresponding communication by the base station and slave device,
respectively.
The base station periodically broadcasts HELLO messages 3261
indicating that it is available for communication. The master
device monitors the HELLO messages during a time period 3263, and,
upon determining that the base does not need servicing, enters a
power conserving mode during a time period 3265. The master then
activates for a time period to monitor the next HELLO message from
the base. If the master has data to send to the base, it transmits
the data during a time period 3271. Likewise, if the base has data
to send to the master, the base transmits the data during a time
period 3269. Once the data is received or sent by the master, it
may again enter a power conserving mode. While HELLO message
protocol is discussed, a number of communication protocols may be
used for communication between the base and the master device. As
may be appreciated, the peripheral LAN master device acts as a
slave to base stations in the premises LAN.
Generally, the communication exchange between the master and the
slave is similar to that described above in reference to FIG. 33b.
Block 3273, however, illustrates a situation where the master
encounters a communication conflict, i.e., it has data to send to
or receive from the slave on the peripheral LAN at the same time it
will monitor the premises LAN for HELLO messages from the base. If
the master has two radio transceivers, the master can service both
networks. If, however, the master only has one radio transceiver,
the master chooses to service one network based on network priority
considerations. For example, in block 3273, it may be desirable to
service the slave because of the presence of data rather than
monitor the premises LAN for HELLO messages from the base. On the
other hand, in block 3275, it may be more desirable to monitor the
premises LAN for HELLO messages rather than transmit an idle sense
message on the peripheral LAN.
FIGS. 35 and 36 are block diagrams illustrating additional power
saving features, wherein ranging and battery parameters are used to
optimally select the appropriate data rate and power level for
subsequent transmissions. Specifically, even though network devices
such as the computer terminal 3007 in FIGS. 28-29 have the
capability of performing high power transmissions, because of
battery power concerns, such devices are configured to utilize
minimum transmission energy. Adjustments are made based on ranging
information and on battery parameters. Similarly, within the
peripheral LAN, even though lower power transceivers are used,
battery conservation issues also justify the use of such data rate
and power adjustments. This process is described in more detail
below in reference to FIGS. 35 and 36.
More specifically, FIG. 35 is a block diagram which illustrates a
protocol 3301 used by a destination peripheral LAN device and a
corresponding protocol 3303 used by a source peripheral LAN device
to adjust the data rate and possibly the power level for future
transmission between the two devices. At a block 3311, upon
receiving a transmission from a source device, the destination
device identifies a range value at a block 3313. In a low cost
embodiment, the range value is identified by considering the
received signal strength indications (RSSI) of the incoming
transmission. Although RSSI circuitry might be placed in all
peripheral LAN radios, the added expense may require that only
peripheral LAN master devices receive the circuitry. This would
mean that only peripheral LAN master devices would perform the
function of the destination device. Other ranging techniques or
signal quality assessments can also be used, such as measuring
jitter in received signals, by adding additional functionality to
the radios. Finally, after identifying the range value at the block
3313, the destination device subsequently transmits the range value
to the slave device from which the transmission was received, at a
block 3314.
Upon receipt of the range value from the destination device at a
block 3321, the source peripheral LAN device evaluates its battery
parameters to identify a subsequent data rate for transmission at a
block 3323. If range value indicates that the destination
peripheral LAN device is very near, the source peripheral LAN
device selects a faster data rate. When the range value indicates a
distant master, the source device selects a slower rate. In this
way, even without adjusting the power level, the total energy
dissipated can be controlled to utilize only that necessary to
carry out the transmission. However, if constraints are placed on
the maximum or minimum data rates, the transmission power may also
need to be modified. For example, to further minimize the
complexity associated with a fully random range of data rate
values, a standard range and set of several data rates may be used.
Under such a scenario, a transmission power adjustment might also
need to supplement the data rate adjustment. Similarly, any
adjustment of power must take into consideration maximum and
minimum operable levels. Data rate adjustment may supplement such
limitations. Any attempted modification of the power and data rate
might take into consideration any available battery parameters such
as those that might indicate a normal or current battery capacity,
the drain on the battery under normal conditions and during
transmission, or the fact that the battery is currently being
charged. The latter parameter proves to be very significant in that
when the battery is being charged, the peripheral LAN slave device
has access to a much greater power source for transmission, which
may justify the highest power transmission and possibly the slowest
data rate under certain circumstances.
Finally, at a block 3325, an indication of the identified data rate
is transmitted back to the destination device so that future
transmissions may take place at the newly selected rate. The
indication of data rate may be explicit in that a message is
transmitted designating the specific rate. Alternately, the data
rate may be transferred implicitly in that the new rate is chose
and used by the source, requiring the destination to adapt to the
change. This might also be done using a predefined header for
synchronization.
In addition, at the block 3325, in another embodiment, along with
the indication of the identified data rate, priority indications
are also be communicated. Whenever battery power is detected as
being low, a radio transmits a higher priority indication, and each
receiver thereafter treats the radio as having a higher protocol
priority than other such radios that exhibit normal power supply
energy. Thus, the remaining battery life is optimized. For example,
in a non-polling network, the low power device might be directly
polled periodically so to allow scheduled wake-ups and contention
free access to a receiver. Similarly, in an alternate embodiment,
priority indications not need to be sent. Instead, the low battery
power device itself exercises protocol priority. For example, for
channel access after detecting that the channel is clear at the end
of an ongoing transmission, devices with normal energy levels are
required to undergo a pseudo-random back-off before attempting a
transmission (to avoid collision). The low power device may either
minimize the back-off period or ignore the back-off period
completely. Thus, the low power device gains channel access easier
than other normal power level devices. Other protocol priority
schemes may also be assigned by the receivers to the low power
device (via the indication), else may be taken directly by the low
power device.
FIG. 36 illustrates an alternate embodiment for carrying out the
data rate and possibly power level adjustment. At a block 3351 upon
binding and possibly periodically, the source peripheral LAN device
sends an indication of its current battery parameters to the
destination peripheral LAN device. This indication may be each of
the parameters or may be an averaged indication of all of the
parameters together. At a block 3355, upon receipt, the destination
peripheral LAN device 355 stores the battery parameters (or
indication). Finally, at a block 3358, upon receiving a
transmission from the source device, based on range determinations
and the stored battery parameters, the destination terminal
identifies the subsequent data rate (and possibly power level).
Thereafter, the new data rate and power level are communicated to
the source device either explicitly or implicitly for future
transmissions.
FIG. 37 illustrates an exemplary block diagram of a radio unit 3501
capable of concurrent participation on multiple LAN's. To transmit,
a control processor 3503 sends a digital data stream to a
modulation encoding circuit 3505. The modulation encoding circuit
3505 encodes the data stream in preparation for modulation by
frequency translation circuit 3507. The carrier frequency used to
translate the data stream is provided by a frequency generator
circuit 3509. Thereafter, the modulated data stream is amplified by
a transmitter amplifier circuit 3511 and then radiated via the one
of a plurality of antennas 3513 that has been selected via an
antenna switching circuit 3515. Together, the modulation encoding
circuitry 3505, translator 3507, amplifier 3511 and associated
support circuitry constitute the transmitter circuitry.
Similarly, to receive data, the RF signal received by the selected
one of the plurality of antennas 3513 is communicated to a receiver
RF processing circuit 3517. After performing a rather coarse
frequency selection, the receiver RF processing circuit 3517
amplifies the RF signal received. The amplified received signal
undergoes a frequency shift to an IF range via a frequency
translation circuit 3519. The frequency translation circuit 3519
provides the center frequency for the frequency shift. Thereafter,
a receiver signal processing circuit receives the IF signal,
performs a more exact channel filtering and demodulation, and
forwards the received data to the control processor 3503, ending
the process. Together, the receiver signal processing 3521,
translator 3517, receiver RF processing 3517 and associated support
circuitry constitute the receiver circuitry.
The control processor 3503 operates pursuant to a set of software
routines stored in memory 3522 which may also store incoming and
outgoing data. Specifically, the memory 3522 contains routines
which define a series of protocols for concurrent communication on
a plurality of LANs. As part of such operation, the control
processor 3503 provides for power savings via a power source
control circuit 3523, i.e., whenever the participating protocols
permit, the control processor 3503 causes selective power down of
the radio transceiver circuitry via a control bus 3525. Also via
the bus 3525, the control processor sets the frequency of the
frequency generator 3509 so as to select the appropriate band and
channel of operation required by a correspondingly selected
protocol. Similarly, the control processor 3503 selects the
appropriate antenna (via the antenna switching circuitry 3515) and
channel filtering in preparation for operation on a selected LAN.
Responding to the software routines stored in the memory 3522, the
control processor 3503 selects the appropriate LANs to establish
participation, detaches from those of the selected LANs in which
participation is no longer needed, identifies from the selected
LANs a current priority LAN in which to actively participate,
maintains a time-shared servicing of the participating LANs.
Further detail regarding this process follows below.
In one embodiment, the control processor 3503 constitutes a typical
microprocessor on an independent integrated circuit. In another
embodiment, the control processor 3503 comprises a combination of
distributed processing circuitry which could be included in a
single integrated circuit as is a typical microprocessor.
Similarly, the memory 3522 could be any type of memory unit(s) or
device(s) capable of software storage.
The radio circuitry illustrated is designed with the frequency
nimble frequency generator 3509 so as to be capable of operation on
a plurality of LANs/WANs. Because each of the plurality may be
allocated different frequency bands, more than one antenna may be
desirable (although a single antenna could be used, antenna
bandwidth limitations might result in an unacceptable
transmission-reception inefficiency). Thus, to select the
appropriate configuration, the control processor 3503 first
identifies the LAN/WAN on which to participate and selects the
corresponding radio configuration parameters from the memory 3521.
Thereafter, using the configuration parameters and pursuant to
control routines stored in the memory 3522, the control processor
3503 sets the frequency of the generator 3509, selects the
appropriate antenna via the antenna switching circuit 3515, and
configures the receiver RF and signal processing circuits 3517 and
3521 for the desired LAN/WAN.
More particularly, the antenna switching circuit 3515 comprises a
plurality of digitally controlled switches, each of which is
associated with one of the plurality of antennas 3513 so as to
permit selective connection by the control processor 3503 of any
available antenna to the transceiver circuitry.
FIG. 38 illustrates an exemplary functional layout of the frequency
generator 3509 of FIG. 37. Basically, the frequency generator 3509
responds to the control processor 3503 by producing the translation
frequency necessary for a selected LAN/WAN. The illustrated
frequency generator comprises a voltage controlled oscillator (VCO)
3601. As is commonly known, for a VCO, the center frequency
F.sub.VCO tracks the input voltage. However, because typical VCO's
are subject to drift, the VCO is stabilized by connecting it in a
phase locked loop to a narrowband reference, such as a crystal
reference oscillator 3603. The oscillator 3603 outputs a signal of
a fixed or reference frequency F.sub.REF to a divide-by-R circuit
3605, which divides as its name implies the reference frequency
F.sub.REF by the known number R. A phase detector 3609 receives the
divided-by-R output of the circuit 3609 and the feedback from the
output of the VCO 3601 via a divide-by-N circuit 3607. Upon
receipt, the phase detector 3609 compares the phase of the outputs
from the circuits 3605 and 3607. Based on the comparison, a phase
error signal is generated and applied to a low-pass loop filter
3611. The output of the filter 3611 is applied to the input of the
VCO 3601 causing the center frequency of the VCO 3601 to lock-in.
Therefore, if the output of the VCO 3601 begins to drift out of
phase of the reference frequency, the phase detector 3609 responds
with a corrective output so as to adjust the center frequency of
the VCO 3601 back in phase.
With the illustrated configuration, the center frequency of the VCO
3601 is a function of the reference frequency as follows:
Thus, to vary the center frequency of the VCO 3601 to correspond to
a band of a selected LAN/WAN in which active participation is
desired, the control processor 3503 (FIG. 37) need only vary the
variables "R" and "N" and perhaps the frequency of the reference
oscillator. Because the output F.sub.REF of the reference
oscillator 3603 is quite stable, the phase lock loop as shown also
keeps the output frequency F.sub.VCO of the VCO 3601 stable.
More specifically, although any other scheme might be implemented,
the value R in the divide-by-R circuit 3605 is chosen so as to
generate an output equal to the channel spacing of a desired
LAN/WAN, while the value N is selected as a multiplying factor for
stepping up the center frequency of the VCO 3601 to the actual
frequency of a given channel. Moreover, the frequency of the
reference oscillator is chosen so as to be divisible by values of R
to yield the channel spacing frequencies of all potential LANs and
WANs. For example, to participate on both MTEL Corporation's Two
Way Paging WAN (operating at 900 MHz with 25 KHz and 50 KHz channel
spacings) and ARDIS Corporation's 800 MHz specialized mobile radio
(SMR) WAN (operating at 25 KHz channel spacings centered at
multiples of 12.5 KHz), a single reference frequency may chosen to
be a whole multiple of 12.5 KHz. Alternately, multiple reference
frequencies may be chosen. Moreover, the value N is chosen to
effectively multiply the output of the divide-by-R circuit 3605 to
the base frequency of a given channel in the selected WAN.
For frequency hopping protocols, the value of R is chosen so as to
yield the spacing between frequency hops. Thus, as N is
incremented, each hopping frequency can be selected. Randomizing
the sequence of such values of N provides a hopping sequence for
use by a base station as described above. Pluralities of hopping
sequences (values of N) may be stored in the memory 3522 (FIG. 37)
for operation on the premises LAN, for example.
In addition to the single port phase locked loop configuration for
the frequency generator 3509, other configurations might also be
implemented. Exemplary circuitry for such configurations can be
found in copending U.S. patent application Ser. No. 08/205,639,
filed Mar. 4, 1994 by Mahany et al., entitled "Method of and
Apparatus For Controlling Modulation of Digital Signals in
Frequency-Modulated Transmissions".
FIG. 39 illustrates further detail of the receiver RF processing
circuit 3517 of FIG. 37. Specifically, a preselector 3651 receives
an incoming RF data signal from a selected one of the plurality of
antennas 3513 (FIG. 37) via an input line 3653. The preselector
3651 provides a bank of passive filters 3657, such as ceramic or
dielectric resonator filters, each of which provides a coarse
filtering for one of the LAN/WAN frequencies to which it is tuned.
One of the outputs from the bank of passive filters 3657 is
selected by the control processor 3503 via a switching circuit 3655
so as to monitor the desired one of the available LANs/WANs.
Thereafter, the selected LAN/WAN RF signal is amplified by an RF
amplifier 3659 before translation by the frequency translation
circuit 3519 (FIG. 37).
FIG. 40 illustrates further detail of the receiver signal
processing circuit 3521 of FIG. 37. In particular, digitally
controlled switching circuits 3701 and 3703 respond to the control
processor 3503 by selecting an appropriate pathway for the
translated IF data signal through one of a bank of IF filters 3705.
Each IF filter is an analog crystal filter, although other types of
filters such as a saw filter might be used. The IF filters 3705
provide rather precise tuning to select the specific channel of a
given LAN/WAN.
After passing through the switching circuit 3703, the filtered IF
data signal is then amplified by an IF amplifier 3707. The
amplified IF signal is then communicated to a demodulator 3709 for
demodulation. The control processor retrieves the incoming
demodulated data signal for processing and potential storage in the
memory 3522 (FIG. 37).
FIG. 41 illustrates further detail of the receiver signal
processing circuit 3521 of FIG. 37. Specifically, the IF signal
resulting from the translation by the frequency translator
circuitry 3519, enters the receiver signal processing circuit via
an input 3751. Thereafter, the IF signal passes through an
anti-aliasing filter 3753, and is amplified by a linear amplifier
3755. An IF oscillator 3757 supplies a reference signal f.sub.REF
for translation of the incoming IF signal at frequency translation
circuits 3759 and 3761. A phase shift circuit 3763 provides for a
90 degree shift of f.sub.REF, i.e., if f.sub.REF is considered a
SINE wave, then the output of the circuit 3763 is the COSINE of
f.sub.REF. Both the SINE and COSINE frequency translation pathways
provide for channel selection of the incoming data signal.
Thereafter the data signals are passed through corresponding low
pass filters 3765 and 3767 in preparation for sampling by analog to
digital (A/D)converters 3769 and 3771. Each A/D converter forwards
the sampled data to a digital signal processor 3773 which provides
for further filtering and demodulation. The digital signal
processor 3773 thereafter forwards the incoming data signal to the
control processor 3503 (FIG. 37) via an output line 3775. Moreover,
although the digital signal processor 3773 and the control
processor 3507 are discrete components in the illustrated example,
they may also be combined into a single integrated circuit.
FIG. 42 illustrates further detail of some of the storage
requirements of the memory 3522 of FIG. 37. To control the radio,
the control processor 3503 (FIG. 37) accesses the information in
the memory 3522 needed for radio setup and operation on a plurality
of LANs/WANs. Among other information, the memory 3522 stores: 1) a
plurality of software protocols, one for each LAN/WAN to be
supported, which define how the radio is to participate on the
corresponding LAN; and 2) an overriding control set of routines
which govern the selection, use and interaction of the plurality of
protocols for participation on desired LANs/WANs.
Specifically, in the memory unit 3522, among other information and
routines, software routines relating to the media access control
(MAC) sublayer of the communication protocol layers can be found.
In general, a MAC sublayer provides detail regarding how
communication generally flows through a corresponding LAN or WAN.
Specifically, the MAC sublayer handles functions such as media
access control, acknowledge, error detection and retransmission.
The MAC layer is fairly independent of the specific radio circuitry
and channel characteristics of the LAN or WAN.
As illustrated, premises LAN, peripheral LAN, vehicular LAN and WAN
MAC routines 3811, 3813, 3815 and 3817 provide definition as to how
the control processor 3503 (FIG. 37) should operate while actively
participating on each LAN or WAN. Although only the several sets of
MAC routines are shown, many other sets might also be stored or
down-loaded into the memory 3522. Moreover, the sets of MAC
routines 3811-17 might also share a set of common routines 3819. In
fact, the sets of MAC routines 3811-17 might be considered a subset
of an overall MAC which shares the common MAC routines 3819.
Below the MAC layer in the communication hierarchy, hardware and
channel related software routines and parameters are necessary for
radio control. For example, such routines govern the specific
switching for channel filtering and antenna selection required by a
given LAN or WAN. Similarly, these routines govern the control
processor 3503's selection of parameters such as for R and N for
the frequency generator 3509 (FIG. 38), or the selective power-down
(via the power source control circuitry 3503--FIG. 37) of portions
or all of the radio circuitry whenever possible to conserve battery
power. As illustrated, such routines and parameters are referred to
as physical (PHY) layer control software 3821. Each of the sets of
MAC routines 3811-17 and 3819 provide specific interaction with the
PHY layer control software 3821.
A set of MAC select/service routines 3823 govern the management of
the overall operation of the radio in the network. For example, if
participation on the premises LAN is desired, the MAC
select/service routines 3823 direct the control processor 3503
(FIG. 37) to the common and premises MAC routines 3819 and 3811
respectively. Thereafter, if concurrent participation with a
peripheral LAN is desired, the select/service routines 3823 direct
the control processor 3503 to enter a sleep mode (if available).
The control processor 3503 refers to the premises LAN MAC routines
3811, and follows the protocol necessary to establish sleep mode on
the premises LAN. Thereafter, the select/service routines 3823
directs the control processor 3503 to the peripheral LAN MAC
routines 3813 to establish and begin servicing the peripheral LAN.
Whenever the peripheral LAN is no longer needed, the select/service
routines 3823 direct a detachment from the peripheral LAN (if
required) as specified in the peripheral LAN MAC routines 3813.
Similarly, if during the servicing of the peripheral LAN a
overriding need to service the premises LAN arises, the processor
3503 is directed to enter a sleep mode via the peripheral LAN MAC
routines 3813, and to return to servicing the premises LAN.
Although not shown, additional protocol layers as well as incoming
and outgoing data are also stored with the memory 3522, which, as
previously articulated, may be a distributed plurality of storage
devices.
FIG. 43 illustrates a software flow chart describing the operation
of the control processor 3503 (FIG. 37) in controlling the radio
unit to participate on multiple LANs. Specifically, at a block
3901, the control processor first determines whether the radio unit
needs to participate on an additional LAN (or WAN). If such
additional participation is needed, at a block 3903, the radio unit
may register sleep mode operation with other participating LANs if
the protocols of those LANs so require and the radio unit has not
already done so. Next, at a block 3905, the control processor
causes the radio unit to poll or scan to locate the desired
additional LAN. If the additional LAN is located at a block 3907,
participation of the radio unit on the additional LAN is
established at a block 3909.
If additional participation is not needed at block 3901, or if the
additional LAN has not been located at block 3907, or once
participation of the radio unit on the additional LAN has been
established at block 3909, the control processor next determines at
a block 3911 whether any of the participating LANs require
servicing. If any given participating LAN requires servicing,, at a
block 3913, the radio unit may be required by the protocol of the
given LAN to reestablish an active participation status on that
LAN, i.e., indicate to the given LAN that the radio unit has ended
the sleep mode. Next, at a block 3915, the radio unit services the
given LAN as needed or until the servicing of another LAN takes
priority over that of the given LAN. At a block 3917, the radio
unit may then be required to register sleep mode operation with the
given LAN if the LAN's protocol so requires.
At that point, or if no participating LAN needs servicing at block
3911, the control processor determines at a block 3919 whether the
radio needs to detach from any given participating LAN. If so, the
radio unit may implicitly detach at a block 3923 if the protocol of
the LAN from which the radio wishes to detach requires no action by
the radio unit. However, at a block 3921, the radio unit may be
required to establish active participation on the LAN in order to
explicitly detach at block 3923. For example, such a situation may
arise when a portable terminal desires to operate on a shorter
range vehicular LAN and detaches from a premises LAN. The portable
terminal may be required by the protocol of the premises LAN to
establish active communication on the premises LAN to permit the
radio unit to inform the premises LAN that it is detaching and can
only be accessed through the vehicular LAN.
Once the radio unit is detached at block 3923, or if the radio unit
does not need to detach from any participating LANs at block 3919,
the control processor returns to block 3901 to again determine
whether the radio unit needs to participate on an additional LAN,
and repeats the process.
FIG. 44 is an alternate embodiment of the software flow chart
wherein the control processor participates on a master LAN and,
when needed, on a slave LAN. Specifically, at a block 3951, the
control processor causes the radio unit to poll or scan in order to
locate the master LAN. If the master LAN has not been located at a
block 3953, polling or scanning for the master LAN continues. Once
the master LAN is located, participation with the master is
established at a block 3955. At a block 3957, the radio unit
participates with the master LAN until the need for the radio unit
to participate on the slave LAN takes precedence. When that
condition occurs, the control processor determines at a block 3959
whether participation of the radio unit on the slave network is
established. If not, such participation is established at a block
3961. Next, at a block 3963, the radio unit services the slave LAN
as needed or until the servicing of the master LAN takes priority.
If the control processor determines at a block 3965 that servicing
of the slave LAN has been completed, the radio unit detaches from
the slave LAN at a block 3967 and returns to block 3957 to continue
participation on the master LAN.
However, if the control processor determines at block 3965 that
servicing has not been, or may not be, completed, the radio unit
does not detach from the slave LAN. In that case, before returning
to block 3957 to service the master LAN, the radio unit may be
required by the protocol of the slave LAN to register sleep mode
operation with the slave LAN at a block 3969.
FIG. 45 illustrates a typical network configuration 4500 operating
in an area where there is a periodic interference source 4501. User
supported terminal 4503 and radio base station 4505 operate
according to the present invention to detect the presence of
periodic interference in their communication attempts and to
optimize their communication procedures so as to tend to
efficiently avoid communication errors caused by the periodic
interference source 4501.
In one embodiment, the periodic interference source 4501 is a
microwave oven, powered by 60 Hz power mains 4507. The sync circuit
4509 of the user supported terminal detects the 60 Hz. signal
radiated from the power mains. The user supported terminal 4503
also has a computer controller 4515, coupled to the, sync circuit
4509, which evaluates RSSI and communication error rates to
determine whether periodic interference is present. If there is
periodic interference, then computer controller 4515 controls
transceiver 4517 to only transmit in the times when the
interference is absent, according to sync circuit 4509.
The sync circuit 4511 of the radio base station may detect the 60
Hz signal directly from the ac power line 4513. The radio base
station 4305 also has a computer controller 4519, coupled to the
sync circuit 4511, which evaluates Received Signal Strength
Indicator (RSSI) and communication error rates to determine whether
periodic interference is present. If there is periodic
interference, then computer controller 4519 controls transceiver
4521 to only transmit in the times when interference is absent,
according to sync circuit 4511.
FIG. 46 is a block diagram of the user supported terminal sync
circuit 4509, in accordance with the present invention. Radiated 60
Hz energy is received by a receiving loop made up of wire 4601
connecting the terminal chassis ground 4603, the ac couple 4605,
the sync generator 4607, and the signal ground 4609. The sync
generator converts the radiated 60 Hz energy into a sync out signal
at 4611, which is sent to computer controller 4515, FIG. 45.
FIG. 47 is a block diagram of the radio base station sync circuit
4511, in accordance with the present invention. The 60 Hz energy
signal is received from the power supply module 4701. The DC line
4703 goes through ac couple 4705 to sync generator 4707, which
converts the 60 Hz. energy into a sync out signal at 4709, which is
sent to computer controller 4519, FIG. 45. The power supply GND and
the power supply chassis GND of component 4701 are grounded via
lines 4711 and 4713, respectively, inside the radio base station's
physical unit enclosure 4715.
FIG. 48 shows a more detailed block diagram of the sync generators
4607 and 4707, according to one embodiment of the present
invention. The sync generators are implemented by an analog
integrated circuit 4800, which receives an ac coupled input 4801
and a DC supply 4803. The integrated circuit 4800 includes a
preamplifier 4805, an active bandpass filter 4807, a limiting
amplifier 4809, and a comparator 4811, and produces a sync out
waveform 4813. FIG. 49 shows a timing diagram on a common time base
for the ac coupled input waveform 4801 and the sync out waveform
4813.
Referring now to FIG. 50, periodic interference 5001, such as a 50%
duty cycle interference wave produced by microwave oven magnetrons
synchronized with the power input waveform for the oven, as shown
with its corresponding sync waveform 5003. Transmission of radio
frequency communication may only take place in the times when
interference is absent, represented by 5005 and 5007. In order to
ensure that communication is not interfered with, additional small
buffer time intervals 5009 are provided before communication begins
and after it ends.
It is possible that periodic interference may force an Access
Interval to begin in one communication transmission time (e.g.
5005) and end after waiting for an absence of interference in a
second communication transmission time (e.g. 5007). The protocol of
the present invention is capable of splitting Access Intervals in
that manner, and may dynamically adjust the timing of an Access
Interval to minimize the effects of such a split on
communication.
FIG. 51 illustrates the process executed by a computer controller
of the present invention to determine whether periodic interference
is present. RSSI and communication packet error rates are monitored
in box 5101. If RSSI is higher than normal at decision box 5103,
then interference may be present. Communication packer error rates
are tested in decision box 5105, where high error rates indicate a
possibility of interference. Next, the sync waveform is evaluated
at the time RSSI is high and packet error rates are high in
decision box 5107. If the sync waveform is active, the detected
interference may be periodic, from the source received by the sync
circuit. Otherwise, the interference may be attributed to some
random interference source, and a standard interference protocol is
implemented.
The computer controller stores in its memory the time of the errors
occurring when the sync waveform is active as represented by box
5109. An evaluation of the timing and frequency of past errors in
decision box 5111. If there is sufficient correlation between the
timing of communication errors and the timing of the sync waveform,
the computer controller will conclude in box 5113 that periodic
interference is present, and will control transmission of messages
according to the sync waveform. When the periodic interference
abates, the computer controller returns the device to normal
communication routines where only directly measured interference
may affect communication timing.
It will be apparent to one skilled in the art having read the
foregoing that various modifications and variations fall within the
scope of the concepts and teachings of this disclosure, and it is
intended to cover all such modifications and variations by the
appended claims.
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